diff --git a/go/pkg/lockfile/compat.go b/go/pkg/lockfile/compat.go
index 9deb9c9..78bda77 100644
--- a/go/pkg/lockfile/compat.go
+++ b/go/pkg/lockfile/compat.go
@@ -53,6 +53,12 @@ func parseInternal(contents []byte, policy *VersionPolicy, paths []string) (File
 	if err := yaml.Unmarshal(contents, &root); err != nil {
 		return File{}, newYAMLParseError(err)
 	}
+	// yaml.v3's Decode rejects duplicate mapping keys, but with a generic
+	// message. Detect duplicate dependency keys first so we can return a
+	// domain-specific, positioned error before the generic decode error fires.
+	if pe := rejectDuplicateDependencyKeys(&root); pe != nil {
+		return File{}, pe
+	}
 	var f File
 	if err := root.Decode(&f); err != nil {
 		return File{}, newYAMLParseError(err)
@@ -477,3 +483,38 @@ func validateKnownFieldsVersioned(f *File, paths []string, version string) *Pars
 	}
 	return nil
 }
+
+// rejectDuplicateDependencyKeys walks the top-level `dependencies` mapping and
+// returns a positioned ParseError on the first duplicate key. yaml.v3's Decode
+// would reject duplicates too, but with a generic message; this yields a
+// domain-specific one. Defensive: any unexpected node shape returns nil and
+// lets the normal decode path handle it.
+func rejectDuplicateDependencyKeys(root *yaml.Node) *ParseError {
+	m := docMapping(root)
+	if m == nil {
+		return nil
+	}
+	_, deps := mappingEntry(m, "dependencies")
+	if deps == nil || deps.Kind != yaml.MappingNode {
+		return nil
+	}
+	seen := make(map[string]int, len(deps.Content)/2)
+	for i := 0; i+1 < len(deps.Content); i += 2 {
+		k := deps.Content[i]
+		// Only compare real string scalar keys. Aliases or non-scalar keys can
+		// share a Value (e.g. empty), so skip them and let Decode surface the
+		// normal shape error rather than a misleading duplicate.
+		if k.Kind != yaml.ScalarNode || k.Tag != "!!str" {
+			continue
+		}
+		if first, dup := seen[k.Value]; dup {
+			return &ParseError{
+				Line:   k.Line,
+				Column: k.Column,
+				Msg:    fmt.Sprintf("duplicate dependency key %q in lockfile dependencies (first defined at line %d)", k.Value, first),
+			}
+		}
+		seen[k.Value] = k.Line
+	}
+	return nil
+}
diff --git a/go/pkg/lockfile/compat_test.go b/go/pkg/lockfile/compat_test.go
index 04cc805..2391a4c 100644
--- a/go/pkg/lockfile/compat_test.go
+++ b/go/pkg/lockfile/compat_test.go
@@ -237,3 +237,45 @@ func TestDefaultPolicy(t *testing.T) {
 	assert.Equal(t, "v0.0.1", p.Min)
 	assert.Equal(t, Version, p.Max)
 }
+
+func TestParse_DuplicateDependencyKey_ConflictingSHAs(t *testing.T) {
+	// Two identical dependency keys with conflicting commit SHAs must be
+	// rejected with a domain-specific, positioned parse error rather than
+	// yaml.v3's generic "mapping key already defined" message.
+	input := `version: 'v0.0.2'
+workflows:
+    '.github/workflows/scenario-duplicate-deps-conflicting-failure.yml':
+        - 'nodeselector/actions-test-fixtures@dup-test'
+dependencies:
+    'nodeselector/actions-test-fixtures@dup-test':
+        ref: 'dup-test'
+        commit: 'sha1-33a384c001ed694ba938667a1d5ace65d6c49de3'
+        owner_id: 29457092
+        repo_id: 1203329948
+    'nodeselector/actions-test-fixtures@dup-test':
+        ref: 'dup-test'
+        commit: 'sha1-0000000000000000000000000000000000000000'
+        owner_id: 29457092
+        repo_id: 1203329948
+`
+	for _, tc := range []struct {
+		name  string
+		parse func() (File, error)
+	}{
+		{"Parse", func() (File, error) { return Parse([]byte(input)) }},
+		{"ParseWithPolicy", func() (File, error) { return ParseWithPolicy([]byte(input), DefaultPolicy()) }},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			_, err := tc.parse()
+			require.Error(t, err)
+			assert.Contains(t, err.Error(), "duplicate dependency key")
+			assert.Contains(t, err.Error(), "nodeselector/actions-test-fixtures@dup-test")
+
+			// Must be the domain path (positioned *ParseError), not a bare
+			// yaml.v3 error — this is what surfaces as a dispatch-time 422.
+			var pe *ParseError
+			require.ErrorAs(t, err, &pe)
+			assert.NotZero(t, pe.Line, "duplicate key error must carry a line position")
+		})
+	}
+}