diff --git a/fuzzing/tools/BUILD b/fuzzing/tools/BUILD
index 9fbcc78..d40cfdb 100644
--- a/fuzzing/tools/BUILD
+++ b/fuzzing/tools/BUILD
@@ -83,3 +83,10 @@ py_test(
     srcs = ["dict_validation_test.py"],
     deps = [":dict_validation"],
 )
+
+py_test(
+    name = "make_corpus_dir_test",
+    srcs = ["make_corpus_dir_test.py"],
+    data = ["make_corpus_dir.py"],
+    deps = [requirement("absl-py")],
+)
diff --git a/fuzzing/tools/make_corpus_dir.py b/fuzzing/tools/make_corpus_dir.py
index aeae9e3..a8b3cc0 100644
--- a/fuzzing/tools/make_corpus_dir.py
+++ b/fuzzing/tools/make_corpus_dir.py
@@ -59,10 +59,22 @@ def main(argv):
                     corpus_line.rstrip("\n"), expanded_file_list)
 
     if expanded_file_list:
+        # Flattening a workspace-relative path with "/" -> "-" is not injective:
+        # distinct inputs such as "corpus/a/b.txt" and "corpus/a-b.txt" both map
+        # to "corpus-a-b.txt". Give each colliding destination a deterministic,
+        # unique name instead of failing the action.
+        used_names = set()
         for corpus in expanded_file_list:
-            dest = os.path.join(FLAGS.output_dir, corpus.replace("/", "-"))
-            # Whatever the separator we choose, there is an chance that
-            # the dest name conflicts with another file
+            base_name = corpus.replace("/", "-")
+            dest_name = base_name
+            counter = 1
+            while dest_name in used_names:
+                dest_name = "%s-%d" % (base_name, counter)
+                counter += 1
+            used_names.add(dest_name)
+
+            dest = os.path.join(FLAGS.output_dir, dest_name)
+            # Guard against an unexpected pre-existing file in the output dir.
             if os.path.exists(dest):
                 print("ERROR: file " + dest + " existed.", file=stderr)
                 return -1
diff --git a/fuzzing/tools/make_corpus_dir_test.py b/fuzzing/tools/make_corpus_dir_test.py
new file mode 100644
index 0000000..f72729a
--- /dev/null
+++ b/fuzzing/tools/make_corpus_dir_test.py
@@ -0,0 +1,171 @@
+# Copyright 2020 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# Lint as: python3
+"""Unit tests for make_corpus_dir.py.
+
+These tests drive the tool through ``--corpus_list_file`` with cleaned,
+workspace-relative paths, matching how ``fuzzing_corpus`` invokes it (see
+``_fuzzing_corpus_impl`` in fuzzing/private/common.bzl, which passes
+``add_all(ctx.files.srcs)`` via a multiline param file).
+"""
+
+import os
+import subprocess
+import sys
+import tempfile
+import unittest
+from pathlib import Path
+
+
+def resolve_script_path():
+    candidates = [Path(__file__).with_name("make_corpus_dir.py")]
+    test_workspace = os.environ.get("TEST_WORKSPACE")
+    if test_workspace:
+        test_srcdir = os.environ.get("TEST_SRCDIR")
+        if test_srcdir:
+            candidates.append(
+                Path(test_srcdir) / test_workspace / "fuzzing" / "tools" /
+                "make_corpus_dir.py")
+        runfiles_dir = os.environ.get("RUNFILES_DIR")
+        if runfiles_dir:
+            candidates.append(
+                Path(runfiles_dir) / test_workspace / "fuzzing" / "tools" /
+                "make_corpus_dir.py")
+
+    for candidate in candidates:
+        if candidate.is_file():
+            return candidate
+
+    manifest_file = os.environ.get("RUNFILES_MANIFEST_FILE")
+    if manifest_file:
+        try:
+            workspace_match = None
+            main_match = None
+            with open(manifest_file, "r", encoding="utf-8") as manifest:
+                for line in manifest:
+                    entry = line.rstrip("\n")
+                    if not entry:
+                        continue
+                    logical_path, separator, real_path = entry.partition(" ")
+                    if not separator:
+                        continue
+                    normalized_path = logical_path.replace("\\", "/")
+                    if not normalized_path.endswith(
+                            "fuzzing/tools/make_corpus_dir.py"):
+                        continue
+                    candidate = Path(real_path)
+                    if not candidate.is_file():
+                        continue
+                    if test_workspace and normalized_path.startswith(
+                            f"{test_workspace}/"):
+                        workspace_match = candidate
+                        break
+                    if normalized_path.startswith("_main/") and not main_match:
+                        main_match = candidate
+            if workspace_match:
+                return workspace_match
+            if main_match:
+                return main_match
+        except OSError:
+            pass
+
+    raise FileNotFoundError(
+        "could not resolve make_corpus_dir.py in test runfiles")
+
+
+SCRIPT_PATH = resolve_script_path()
+
+
+class MakeCorpusDirTest(unittest.TestCase):
+
+    def run_tool_with_corpus_list_file(self, corpus_paths, cwd):
+        """Invokes the tool the way fuzzing_corpus does: a param file of
+        cleaned, workspace-relative paths passed via --corpus_list_file."""
+        params = cwd / "corpus_params.txt"
+        params.write_text("\n".join(corpus_paths) + "\n", encoding="utf-8")
+        return subprocess.run(
+            [
+                sys.executable,
+                str(SCRIPT_PATH),
+                f"--corpus_list_file={params.name}",
+                "--output_dir=out",
+            ],
+            cwd=str(cwd),
+            text=True,
+            capture_output=True,
+            check=False,
+        )
+
+    def _write(self, root, rel_path, contents):
+        path = root / rel_path
+        path.parent.mkdir(parents=True, exist_ok=True)
+        path.write_text(contents, encoding="utf-8")
+        return str(rel_path).replace(os.sep, "/")
+
+    def test_copies_distinct_files_without_collision(self):
+        with tempfile.TemporaryDirectory() as td:
+            tmp = Path(td)
+            a = self._write(tmp, Path("corpus") / "a.txt", "A")
+            b = self._write(tmp, Path("corpus") / "nested" / "b.txt", "B")
+
+            result = self.run_tool_with_corpus_list_file([a, b], cwd=tmp)
+
+            self.assertEqual(result.returncode, 0, msg=result.stderr)
+            copied = [p for p in (tmp / "out").iterdir() if p.is_file()]
+            self.assertEqual(len(copied), 2)
+            self.assertEqual(
+                sorted(p.read_text(encoding="utf-8") for p in copied),
+                ["A", "B"])
+
+    def test_flatten_collision_is_disambiguated(self):
+        # Reachable collision: "corpus/a/b.txt" and "corpus/a-b.txt" both
+        # flatten to "corpus-a-b.txt" under replace("/", "-"). On current main
+        # this aborts the action with "ERROR: file ... existed." / return -1.
+        with tempfile.TemporaryDirectory() as td:
+            tmp = Path(td)
+            nested = self._write(tmp, Path("corpus") / "a" / "b.txt", "NESTED")
+            flat = self._write(tmp, Path("corpus") / "a-b.txt", "FLAT")
+
+            result = self.run_tool_with_corpus_list_file([nested, flat], cwd=tmp)
+
+            self.assertEqual(result.returncode, 0, msg=result.stderr)
+            copied = [p for p in (tmp / "out").iterdir() if p.is_file()]
+            self.assertEqual(len(copied), 2)
+            self.assertEqual(
+                sorted(p.read_text(encoding="utf-8") for p in copied),
+                ["FLAT", "NESTED"])
+
+    def test_flatten_collision_variant_is_disambiguated(self):
+        # A second reachable variant where the "/" boundary in one path aligns
+        # with a literal "-" in another: "a/b/c.txt" and "a-b/c.txt" both
+        # flatten to "a-b-c.txt".
+        with tempfile.TemporaryDirectory() as td:
+            tmp = Path(td)
+            first = self._write(tmp, Path("a") / "b" / "c.txt", "FIRST")
+            second = self._write(tmp, Path("a-b") / "c.txt", "SECOND")
+
+            result = self.run_tool_with_corpus_list_file(
+                [first, second], cwd=tmp)
+
+            self.assertEqual(result.returncode, 0, msg=result.stderr)
+            copied = [p for p in (tmp / "out").iterdir() if p.is_file()]
+            self.assertEqual(len(copied), 2)
+            self.assertEqual(
+                sorted(p.read_text(encoding="utf-8") for p in copied),
+                ["FIRST", "SECOND"])
+
+
+if __name__ == "__main__":
+    unittest.main()