diff --git a/.github/workflows/opencode-review.yml b/.github/workflows/opencode-review.yml
index 4980e30f..da8c08c2 100644
--- a/.github/workflows/opencode-review.yml
+++ b/.github/workflows/opencode-review.yml
@@ -638,6 +638,11 @@ jobs:
               | sort -u
           }
 
+          javascript_test_script_collects_coverage() {
+            jq -e '(.scripts.test // "") | test("(^|[[:space:]])--coverage([.=[:space:]]|$)|c8([[:space:]]|$)|nyc([[:space:]]|$)")' \
+              package.json >/dev/null 2>&1
+          }
+
           declared_package_manager() {
             if [ -f package.json ]; then
               jq -r '.packageManager // "" | split("@")[0]' package.json 2>/dev/null || true
@@ -1388,11 +1393,19 @@ jobs:
                 run_and_capture "JavaScript/TypeScript coverage script" "$package_runner" run coverage
                 javascript_coverage_ran=1
               elif jq -e '.scripts.test // empty' package.json >/dev/null; then
-                case "$package_runner" in
-                  npm) run_and_capture "JavaScript/TypeScript test coverage" npm test -- --coverage ;;
-                  pnpm) run_and_capture "JavaScript/TypeScript test coverage" pnpm test -- --coverage ;;
-                  yarn) run_and_capture "JavaScript/TypeScript test coverage" yarn test --coverage ;;
-                esac
+                if javascript_test_script_collects_coverage; then
+                  case "$package_runner" in
+                    npm) run_and_capture "JavaScript/TypeScript test coverage" npm test ;;
+                    pnpm) run_and_capture "JavaScript/TypeScript test coverage" pnpm test ;;
+                    yarn) run_and_capture "JavaScript/TypeScript test coverage" yarn test ;;
+                  esac
+                else
+                  case "$package_runner" in
+                    npm) run_and_capture "JavaScript/TypeScript test coverage" npm test -- --coverage ;;
+                    pnpm) run_and_capture "JavaScript/TypeScript test coverage" pnpm test -- --coverage ;;
+                    yarn) run_and_capture "JavaScript/TypeScript test coverage" yarn test --coverage ;;
+                  esac
+                fi
                 javascript_coverage_ran=1
               else
                 append "### JavaScript/TypeScript test coverage"
diff --git a/tests/test_opencode_agent_contract.py b/tests/test_opencode_agent_contract.py
index 8e18d152..fb1b2a55 100644
--- a/tests/test_opencode_agent_contract.py
+++ b/tests/test_opencode_agent_contract.py
@@ -278,6 +278,27 @@ def test_opencode_coverage_prefers_declared_pnpm_runner_before_npm():
     assert "return" in declared_pnpm_block
 
 
+def test_opencode_coverage_does_not_duplicate_existing_javascript_coverage():
+    """An existing coverage flag/tool must run once instead of receiving a duplicate flag."""
+    workflow = Path(".github/workflows/opencode-review.yml").read_text(encoding="utf-8")
+    measure_start = workflow.index("      - name: Measure test and docstring evidence\n")
+    measure_end = workflow.index("\n      - name:", measure_start + 1)
+    measure_step = workflow[measure_start:measure_end]
+
+    assert "javascript_test_script_collects_coverage()" in measure_step
+    assert "if javascript_test_script_collects_coverage; then" in measure_step
+    assert (
+        'npm) run_and_capture "JavaScript/TypeScript test coverage" npm test ;;'
+        in measure_step
+    )
+    assert (
+        'npm) run_and_capture "JavaScript/TypeScript test coverage" npm test -- --coverage ;;'
+        in measure_step
+    )
+    assert 'test("(^|[[:space:]])--coverage([.=[:space:]]|$)' in measure_step
+    assert '|c8([[:space:]]|$)|nyc([[:space:]]|$)")' in measure_step
+
+
 def test_opencode_coverage_discovers_changed_nested_javascript_package(tmp_path):
     """A changed JS file must select its nearest nested package.json for coverage."""
     bash = shutil.which("bash")