diff --git a/tests/lint.bats b/tests/lint.bats
index f0b0306..ac19dfe 100644
--- a/tests/lint.bats
+++ b/tests/lint.bats
@@ -69,3 +69,80 @@ EOF
   run bash "$SKILL_SCRIPTS/scoped-lint.sh" genome-test wiki/sources/good.md
   [ "$status" -eq 0 ]
 }
+
+# --- duplicate-slug advisory (check_duplicates + its distance helpers) --------
+# These guard the dedup feature: correct edit-distance math, the warn-only
+# contract, the exact-self-match skip (run-ingest appends new slugs to the
+# index before lint runs), and that the advisory fires once per run, not once
+# per file.
+
+@test "levenshtein: identical strings have distance 0" {
+  run levenshtein cat cat
+  [ "$status" -eq 0 ]
+  [ "$output" -eq 0 ]
+}
+
+@test "levenshtein: kitten→sitting is 3 (textbook case)" {
+  run levenshtein kitten sitting
+  [ "$output" -eq 3 ]
+}
+
+@test "similarity: identical strings score 100" {
+  run similarity gpu-pricing gpu-pricing
+  [ "$output" -eq 100 ]
+}
+
+@test "check_duplicates: warns on a near-duplicate of an indexed concept" {
+  G="$(make_fixture_genome)"; cd "$G"
+  printf -- '- [[concepts/llm-routing]] — x\n' >> wiki/index.md
+  cat > .ingest-manifest.json <<'JSON'
+{"raw_source":"src","pages":[{"path":"wiki/concepts/llm-routings.md","status":"created"}]}
+JSON
+  run check_duplicates .ingest-manifest.json
+  [ "$status" -eq 0 ]
+  [[ "$output" == *"≈"* ]]
+  [[ "$output" == *"llm-routings"* ]]
+}
+
+@test "check_duplicates: silent when the new slug is unlike anything indexed" {
+  G="$(make_fixture_genome)"; cd "$G"
+  printf -- '- [[concepts/llm-routing]] — x\n' >> wiki/index.md
+  cat > .ingest-manifest.json <<'JSON'
+{"raw_source":"src","pages":[{"path":"wiki/concepts/budget-hardware.md","status":"created"}]}
+JSON
+  run check_duplicates .ingest-manifest.json
+  [ "$status" -eq 0 ]
+  [[ "$output" != *"≈"* ]]
+}
+
+@test "check_duplicates: an exact self-match is not flagged (index already has the slug)" {
+  G="$(make_fixture_genome)"; cd "$G"
+  # run-ingest step 1 inserts this run's slug into the index BEFORE lint runs;
+  # the slug must not be reported as a duplicate of itself.
+  printf -- '- [[concepts/llm-routing]] — x\n' >> wiki/index.md
+  cat > .ingest-manifest.json <<'JSON'
+{"raw_source":"src","pages":[{"path":"wiki/concepts/llm-routing.md","status":"created"}]}
+JSON
+  run check_duplicates .ingest-manifest.json
+  [ "$status" -eq 0 ]
+  [[ "$output" != *"≈"* ]]
+}
+
+@test "scoped-lint: duplicate advisory fires once across multiple files, not per file" {
+  G="$(make_fixture_genome)"
+  write_page "$G/wiki/concepts/data-pipelines.md" concept genome-test
+  write_page "$G/wiki/concepts/other-topic.md"    concept genome-test
+  printf -- '- [[concepts/data-pipeline]] — x\n' >> "$G/wiki/index.md"
+  cat > "$G/.ingest-manifest.json" <<'JSON'
+{"raw_source":"src","pages":[
+  {"path":"wiki/concepts/data-pipelines.md","status":"created"},
+  {"path":"wiki/concepts/other-topic.md","status":"created"}
+]}
+JSON
+  cd "$G"
+  export KG_LIB_DIR="$LIB_DIR"
+  run bash "$SKILL_SCRIPTS/scoped-lint.sh" genome-test \
+      wiki/concepts/data-pipelines.md wiki/concepts/other-topic.md
+  [ "$status" -eq 0 ]
+  [ "$(grep -c "≈" <<< "$output")" -eq 1 ]
+}