diff --git a/packages/beacon-node/src/api/impl/beacon/blocks/index.ts b/packages/beacon-node/src/api/impl/beacon/blocks/index.ts
index 8a2c82192618..711d199f94e5 100644
--- a/packages/beacon-node/src/api/impl/beacon/blocks/index.ts
+++ b/packages/beacon-node/src/api/impl/beacon/blocks/index.ts
@@ -40,7 +40,13 @@ import {ImportBlockOpts} from "../../../../chain/blocks/types.js";
 import {verifyBlocksInEpoch} from "../../../../chain/blocks/verifyBlock.js";
 import {BeaconChain} from "../../../../chain/chain.js";
 import {ChainEvent} from "../../../../chain/emitter.js";
-import {BlockError, BlockErrorCode, BlockGossipError} from "../../../../chain/errors/index.js";
+import {
+  BlockError,
+  BlockErrorCode,
+  BlockGossipError,
+  ExecutionPayloadEnvelopeError,
+  ExecutionPayloadEnvelopeErrorCode,
+} from "../../../../chain/errors/index.js";
 import {
   BlockType,
   ProduceFullBellatrix,
@@ -683,7 +689,34 @@ export function getBeaconBlockApi({
         throw new ApiError(400, `Envelope slot ${slot} does not match block slot ${block.slot}`);
       }
 
-      await validateApiExecutionPayloadEnvelope(chain, signedExecutionPayloadEnvelope);
+      try {
+        await validateApiExecutionPayloadEnvelope(chain, signedExecutionPayloadEnvelope);
+      } catch (e) {
+        // idempotent behavior towards an already received envelope
+        if (
+          e instanceof ExecutionPayloadEnvelopeError &&
+          e.type.code === ExecutionPayloadEnvelopeErrorCode.ENVELOPE_ALREADY_KNOWN
+        ) {
+          const existing = chain.seenPayloadEnvelopeInputCache.get(blockRootHex);
+          const existingEnvelope = existing?.hasPayloadEnvelope() ? existing.getPayloadEnvelope() : undefined;
+          const existingBlockHashHex = existingEnvelope && toRootHex(existingEnvelope.message.payload.blockHash);
+          if (existingBlockHashHex !== undefined && existingBlockHashHex !== blockHashHex) {
+            chain.logger.warn("Execution payload envelope already known with a DIFFERENT block hash", {
+              slot,
+              blockRoot: blockRootHex,
+              existingBlockHash: existingBlockHashHex,
+              newBlockHash: blockHashHex,
+            });
+          } else {
+            chain.logger.debug("Execution payload envelope already known, skipping duplicate", {
+              slot,
+              blockRoot: blockRootHex,
+            });
+          }
+          return; // 200 OK
+        }
+        throw e;
+      }
 
       const isSelfBuild = envelope.builderIndex === BUILDER_INDEX_SELF_BUILD;
       let dataColumnSidecars: gloas.DataColumnSidecar[] = [];
@@ -732,12 +765,14 @@ export function getBeaconBlockApi({
         throw new ApiError(404, `PayloadEnvelopeInput not found for block root ${blockRootHex}`);
       }
 
-      payloadInput.addPayloadEnvelope({
-        envelope: signedExecutionPayloadEnvelope,
-        source: PayloadEnvelopeInputSource.api,
-        seenTimestampSec,
-        peerIdStr: undefined,
-      });
+      if (!payloadInput.hasPayloadEnvelope()) {
+        payloadInput.addPayloadEnvelope({
+          envelope: signedExecutionPayloadEnvelope,
+          source: PayloadEnvelopeInputSource.api,
+          seenTimestampSec,
+          peerIdStr: undefined,
+        });
+      }
 
       if (dataColumnSidecars.length > 0) {
         for (const columnSidecar of dataColumnSidecars) {
diff --git a/packages/beacon-node/test/unit/api/impl/beacon/blocks/publishExecutionPayloadEnvelope.test.ts b/packages/beacon-node/test/unit/api/impl/beacon/blocks/publishExecutionPayloadEnvelope.test.ts
new file mode 100644
index 000000000000..69ee4bbd1037
--- /dev/null
+++ b/packages/beacon-node/test/unit/api/impl/beacon/blocks/publishExecutionPayloadEnvelope.test.ts
@@ -0,0 +1,227 @@
+import {afterEach, beforeEach, describe, expect, it, vi} from "vitest";
+import {when} from "vitest-when";
+import {createChainForkConfig, defaultChainConfig} from "@lodestar/config";
+import {PayloadStatus} from "@lodestar/fork-choice";
+import {ssz} from "@lodestar/types";
+import {toRootHex} from "@lodestar/utils";
+import {getBeaconBlockApi} from "../../../../../../src/api/impl/beacon/blocks/index.js";
+import {
+  ExecutionPayloadEnvelopeError,
+  ExecutionPayloadEnvelopeErrorCode,
+  GossipAction,
+} from "../../../../../../src/chain/errors/index.js";
+import {validateApiExecutionPayloadEnvelope} from "../../../../../../src/chain/validation/executionPayloadEnvelope.js";
+import {ApiTestModules, getApiTestModules} from "../../../../../utils/api.js";
+import {generateProtoBlock} from "../../../../../utils/typeGenerator.js";
+
+vi.mock("../../../../../../src/chain/validation/executionPayloadEnvelope.js", async (importActual) => {
+  const mod = await importActual<typeof import("../../../../../../src/chain/validation/executionPayloadEnvelope.js")>();
+  return {...mod, validateApiExecutionPayloadEnvelope: vi.fn()};
+});
+
+const gloasConfig = createChainForkConfig({
+  ...defaultChainConfig,
+  GLOAS_FORK_EPOCH: 0,
+});
+
+describe("api - beacon - publishExecutionPayloadEnvelope", () => {
+  let modules: ApiTestModules;
+  let api: ReturnType<typeof getBeaconBlockApi>;
+
+  const slot = 1;
+  const blockRoot = Buffer.alloc(32, 1);
+  const blockHash = Buffer.alloc(32, 2);
+  const blockRootHex = toRootHex(blockRoot);
+  const blockHashHex = toRootHex(blockHash);
+
+  function buildEnvelope(beaconBlockRoot: Uint8Array, payloadBlockHash: Uint8Array) {
+    const signed = ssz.gloas.SignedExecutionPayloadEnvelope.defaultValue();
+    signed.message.beaconBlockRoot = beaconBlockRoot;
+    signed.message.payload.slotNumber = slot;
+    signed.message.payload.blockHash = payloadBlockHash;
+    return signed;
+  }
+
+  const signedEnvelope = buildEnvelope(blockRoot, blockHash);
+
+  function makeInput(opts: {hasEnvelope: boolean; storedEnvelope?: ReturnType<typeof buildEnvelope>}) {
+    return {
+      hasPayloadEnvelope: vi.fn().mockReturnValue(opts.hasEnvelope),
+      getPayloadEnvelope: vi.fn().mockReturnValue(opts.storedEnvelope ?? signedEnvelope),
+      addPayloadEnvelope: vi.fn(),
+      addColumn: vi.fn(),
+    };
+  }
+
+  function throwAlreadyKnown(): void {
+    vi.mocked(validateApiExecutionPayloadEnvelope).mockRejectedValue(
+      new ExecutionPayloadEnvelopeError(GossipAction.IGNORE, {
+        code: ExecutionPayloadEnvelopeErrorCode.ENVELOPE_ALREADY_KNOWN,
+        blockRoot: blockRootHex,
+        slot,
+      })
+    );
+  }
+
+  beforeEach(() => {
+    modules = getApiTestModules({config: gloasConfig});
+    api = getBeaconBlockApi(modules);
+
+    when(modules.forkChoice.getBlockHex)
+      .calledWith(blockRootHex, PayloadStatus.EMPTY)
+      .thenReturn(generateProtoBlock({slot}));
+
+    (modules.network as any).publishSignedExecutionPayloadEnvelope = vi.fn().mockResolvedValue(1);
+    modules.chain.processExecutionPayload = vi.fn().mockResolvedValue(undefined) as any;
+  });
+
+  afterEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it("happy path: unknown envelope is added and published (200)", async () => {
+    vi.mocked(validateApiExecutionPayloadEnvelope).mockResolvedValue(undefined);
+    const input = makeInput({hasEnvelope: false});
+    vi.mocked(modules.chain.seenPayloadEnvelopeInputCache).get.mockReturnValue(input as any);
+
+    await expect(
+      api.publishExecutionPayloadEnvelope({signedExecutionPayloadEnvelope: signedEnvelope})
+    ).resolves.toBeUndefined();
+
+    expect(input.addPayloadEnvelope).toHaveBeenCalledTimes(1);
+    expect(modules.chain.processExecutionPayload).toHaveBeenCalledTimes(1);
+    expect(modules.network.publishSignedExecutionPayloadEnvelope).toHaveBeenCalledTimes(1);
+  });
+
+  it("VC retry: envelope already in cache → 200, not added again", async () => {
+    throwAlreadyKnown();
+    const input = makeInput({hasEnvelope: true}); // same block hash as incoming
+    vi.mocked(modules.chain.seenPayloadEnvelopeInputCache).get.mockReturnValue(input as any);
+
+    await expect(
+      api.publishExecutionPayloadEnvelope({signedExecutionPayloadEnvelope: signedEnvelope})
+    ).resolves.toBeUndefined();
+
+    expect(input.addPayloadEnvelope).not.toHaveBeenCalled();
+    expect(modules.chain.processExecutionPayload).not.toHaveBeenCalled();
+    expect(modules.network.publishSignedExecutionPayloadEnvelope).not.toHaveBeenCalled();
+    expect(modules.chain.logger.debug).toHaveBeenCalledWith(
+      "Execution payload envelope already known, skipping duplicate",
+      expect.objectContaining({slot, blockRoot: blockRootHex})
+    );
+    expect(modules.chain.logger.warn).not.toHaveBeenCalled();
+  });
+
+  it("gossip-first: already known, cache pruned → 200 (null-safe lookup)", async () => {
+    throwAlreadyKnown();
+    vi.mocked(modules.chain.seenPayloadEnvelopeInputCache).get.mockReturnValue(undefined); // pruned
+
+    await expect(
+      api.publishExecutionPayloadEnvelope({signedExecutionPayloadEnvelope: signedEnvelope})
+    ).resolves.toBeUndefined();
+
+    expect(modules.chain.processExecutionPayload).not.toHaveBeenCalled();
+    expect(modules.network.publishSignedExecutionPayloadEnvelope).not.toHaveBeenCalled();
+    expect(modules.chain.logger.debug).toHaveBeenCalledWith(
+      "Execution payload envelope already known, skipping duplicate",
+      expect.objectContaining({slot, blockRoot: blockRootHex})
+    );
+    expect(modules.chain.logger.warn).not.toHaveBeenCalled();
+  });
+
+  it("divergent hash: known envelope with a DIFFERENT block hash → warn + 200", async () => {
+    throwAlreadyKnown();
+    const stored = buildEnvelope(blockRoot, Buffer.alloc(32, 0xff));
+    const input = makeInput({hasEnvelope: true, storedEnvelope: stored});
+    vi.mocked(modules.chain.seenPayloadEnvelopeInputCache).get.mockReturnValue(input as any);
+
+    await expect(
+      api.publishExecutionPayloadEnvelope({signedExecutionPayloadEnvelope: signedEnvelope})
+    ).resolves.toBeUndefined();
+
+    expect(modules.chain.processExecutionPayload).not.toHaveBeenCalled();
+    expect(modules.network.publishSignedExecutionPayloadEnvelope).not.toHaveBeenCalled();
+    expect(input.addPayloadEnvelope).not.toHaveBeenCalled();
+    expect(modules.chain.logger.warn).toHaveBeenCalledWith(
+      "Execution payload envelope already known with a DIFFERENT block hash",
+      expect.objectContaining({
+        slot,
+        blockRoot: blockRootHex,
+        existingBlockHash: toRootHex(stored.message.payload.blockHash),
+        newBlockHash: blockHashHex,
+      })
+    );
+  });
+
+  it("race path: envelope set between validation and the guard → not added twice, still published", async () => {
+    vi.mocked(validateApiExecutionPayloadEnvelope).mockResolvedValue(undefined);
+    const input = makeInput({hasEnvelope: true});
+    vi.mocked(modules.chain.seenPayloadEnvelopeInputCache).get.mockReturnValue(input as any);
+
+    await expect(
+      api.publishExecutionPayloadEnvelope({signedExecutionPayloadEnvelope: signedEnvelope})
+    ).resolves.toBeUndefined();
+
+    expect(input.addPayloadEnvelope).not.toHaveBeenCalled();
+    expect(modules.chain.processExecutionPayload).toHaveBeenCalledTimes(1);
+    expect(modules.network.publishSignedExecutionPayloadEnvelope).toHaveBeenCalledTimes(1);
+  });
+
+  it("propagates a non-ENVELOPE_ALREADY_KNOWN validation error (different code)", async () => {
+    vi.mocked(validateApiExecutionPayloadEnvelope).mockRejectedValue(
+      new ExecutionPayloadEnvelopeError(GossipAction.REJECT, {
+        code: ExecutionPayloadEnvelopeErrorCode.INVALID_SIGNATURE,
+      })
+    );
+    const input = makeInput({hasEnvelope: false});
+    vi.mocked(modules.chain.seenPayloadEnvelopeInputCache).get.mockReturnValue(input as any);
+
+    await expect(
+      api.publishExecutionPayloadEnvelope({signedExecutionPayloadEnvelope: signedEnvelope})
+    ).rejects.toThrow();
+
+    expect(input.addPayloadEnvelope).not.toHaveBeenCalled();
+    expect(modules.chain.processExecutionPayload).not.toHaveBeenCalled();
+    expect(modules.network.publishSignedExecutionPayloadEnvelope).not.toHaveBeenCalled();
+  });
+
+  it("propagates a non-envelope validation error (plain Error)", async () => {
+    vi.mocked(validateApiExecutionPayloadEnvelope).mockRejectedValue(new Error("boom"));
+    const input = makeInput({hasEnvelope: false});
+    vi.mocked(modules.chain.seenPayloadEnvelopeInputCache).get.mockReturnValue(input as any);
+
+    await expect(
+      api.publishExecutionPayloadEnvelope({signedExecutionPayloadEnvelope: signedEnvelope})
+    ).rejects.toThrow();
+
+    expect(input.addPayloadEnvelope).not.toHaveBeenCalled();
+    expect(modules.chain.processExecutionPayload).not.toHaveBeenCalled();
+    expect(modules.network.publishSignedExecutionPayloadEnvelope).not.toHaveBeenCalled();
+  });
+
+  it("rejects 404 when the block is unknown (before validation)", async () => {
+    when(modules.forkChoice.getBlockHex).calledWith(blockRootHex, PayloadStatus.EMPTY).thenReturn(null);
+
+    await expect(
+      api.publishExecutionPayloadEnvelope({signedExecutionPayloadEnvelope: signedEnvelope})
+    ).rejects.toMatchObject({statusCode: 404});
+
+    expect(validateApiExecutionPayloadEnvelope).not.toHaveBeenCalled();
+    expect(modules.chain.processExecutionPayload).not.toHaveBeenCalled();
+    expect(modules.network.publishSignedExecutionPayloadEnvelope).not.toHaveBeenCalled();
+  });
+
+  it("rejects 400 on slot mismatch (before validation)", async () => {
+    when(modules.forkChoice.getBlockHex)
+      .calledWith(blockRootHex, PayloadStatus.EMPTY)
+      .thenReturn(generateProtoBlock({slot: slot + 1}));
+
+    await expect(
+      api.publishExecutionPayloadEnvelope({signedExecutionPayloadEnvelope: signedEnvelope})
+    ).rejects.toMatchObject({statusCode: 400});
+
+    expect(validateApiExecutionPayloadEnvelope).not.toHaveBeenCalled();
+    expect(modules.chain.processExecutionPayload).not.toHaveBeenCalled();
+    expect(modules.network.publishSignedExecutionPayloadEnvelope).not.toHaveBeenCalled();
+  });
+});