diff --git a/backend/src/database/migrations/V1775000000__fix-member-segment-affiliations.sql b/backend/src/database/migrations/V1775000000__fix-member-segment-affiliations.sql
new file mode 100644
index 0000000000..ce04336c47
--- /dev/null
+++ b/backend/src/database/migrations/V1775000000__fix-member-segment-affiliations.sql
@@ -0,0 +1,24 @@
+-- 1. Deduplicate existing memberSegmentAffiliations
+DELETE FROM "memberSegmentAffiliations" a USING (
+    SELECT MIN(id) as keep_id, "memberId", "segmentId", "organizationId", "dateStart", "dateEnd"
+    FROM "memberSegmentAffiliations"
+    GROUP BY "memberId", "segmentId", "organizationId", "dateStart", "dateEnd"
+    HAVING COUNT(*) > 1
+) b
+WHERE a."memberId" = b."memberId"
+AND a."segmentId" = b."segmentId"
+AND a."organizationId" IS NOT DISTINCT FROM b."organizationId"
+AND a."dateStart" IS NOT DISTINCT FROM b."dateStart"
+AND a."dateEnd" IS NOT DISTINCT FROM b."dateEnd"
+AND a.id <> b.keep_id;
+
+-- 2. Add an index to prevent exact duplicates in the future
+-- Using COALESCE ensures that NULL values are logically treated as equal 
+-- across all supported PostgreSQL versions for the sake of uniqueness.
+CREATE UNIQUE INDEX "uq_member_segment_affiliations" ON "memberSegmentAffiliations" (
+    "memberId",
+    "segmentId",
+    COALESCE("organizationId", '00000000-0000-0000-0000-000000000000'::uuid),
+    COALESCE("dateStart", '1970-01-01T00:00:00Z'::timestamp),
+    COALESCE("dateEnd", '1970-01-01T00:00:00Z'::timestamp)
+);
diff --git a/backend/src/database/repositories/memberSegmentAffiliationRepository.ts b/backend/src/database/repositories/memberSegmentAffiliationRepository.ts
index b55bf6d5ec..c838db3cb1 100644
--- a/backend/src/database/repositories/memberSegmentAffiliationRepository.ts
+++ b/backend/src/database/repositories/memberSegmentAffiliationRepository.ts
@@ -44,6 +44,32 @@ class MemberSegmentAffiliationRepository extends RepositoryBase<
 
     const transaction = this.transaction
 
+    const existing = await this.options.database.sequelize.query(
+      `SELECT "id" FROM "memberSegmentAffiliations"
+       WHERE "memberId" = :memberId
+       AND "segmentId" = :segmentId
+       AND "organizationId" IS NOT DISTINCT FROM :organizationId
+       AND "dateStart" IS NOT DISTINCT FROM :dateStart
+       AND "dateEnd" IS NOT DISTINCT FROM :dateEnd
+       LIMIT 1`,
+      {
+        replacements: {
+          memberId: data.memberId,
+          segmentId: data.segmentId,
+          organizationId: data.organizationId,
+          dateStart: data.dateStart || null,
+          dateEnd: data.dateEnd || null,
+        },
+        type: QueryTypes.SELECT,
+        transaction,
+      }
+    )
+
+    if (existing.length > 0) {
+      await this.updateAffiliation(data.memberId, data.segmentId, data.organizationId)
+      return this.findById((existing[0] as any).id)
+    }
+
     const affiliationInsertResult = await this.options.database.sequelize.query(
       `INSERT INTO "memberSegmentAffiliations" ("id", "memberId", "segmentId", "organizationId", "dateStart", "dateEnd")
           VALUES