diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index aecbca9..06db624 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -165,7 +165,7 @@ jobs:
       - name: Verify output
         run: |
           echo "Updated files: ${{ steps.patch.outputs.updated_files }}"
-          
+
           # Output should contain both files
           if [[ "${{ steps.patch.outputs.updated_files }}" == *"app1/.env"* ]] && \
              [[ "${{ steps.patch.outputs.updated_files }}" == *"app2/.env"* ]]; then
@@ -173,4 +173,63 @@ jobs:
           else
             echo "Error: Output doesn't contain expected files"
             exit 1
-          fi
\ No newline at end of file
+          fi
+
+  test-key-only-lines:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Create env file with key-only lines
+        run: |
+          mkdir -p test
+          cat > test/.env <<EOF
+          DATABASE_URL=postgres://localhost
+          SENTRY_RELEASE
+          API_KEY=secret123
+          FEATURE_FLAG
+          EOF
+
+          echo "Original test/.env:"
+          cat test/.env
+
+      - name: Patch file with key-only lines
+        uses: ./
+        with:
+          path: test
+          patches: |
+            {
+              ".env": {
+                "SENTRY_RELEASE": "v1.2.3",
+                "FEATURE_FLAG": "enabled",
+                "NEW_KEY": "new_value"
+              }
+            }
+
+      - name: Verify key-only lines are replaced, not duplicated
+        run: |
+          echo "Patched test/.env:"
+          cat test/.env
+
+          # SENTRY_RELEASE should appear exactly once with the new value
+          if [ $(grep -c "^SENTRY_RELEASE" test/.env) -ne 1 ]; then
+            echo "Error: SENTRY_RELEASE appears $(grep -c "^SENTRY_RELEASE" test/.env) times (expected 1)"
+            exit 1
+          fi
+          grep -q "^SENTRY_RELEASE=v1.2.3$" test/.env || exit 1
+
+          # FEATURE_FLAG should appear exactly once with the new value
+          if [ $(grep -c "^FEATURE_FLAG" test/.env) -ne 1 ]; then
+            echo "Error: FEATURE_FLAG appears $(grep -c "^FEATURE_FLAG" test/.env) times (expected 1)"
+            exit 1
+          fi
+          grep -q "^FEATURE_FLAG=enabled$" test/.env || exit 1
+
+          # Existing keys should still be present
+          grep -q "DATABASE_URL=postgres://localhost" test/.env || exit 1
+          grep -q "API_KEY=secret123" test/.env || exit 1
+
+          # New key should be added
+          grep -q "NEW_KEY=new_value" test/.env || exit 1
+
+          echo "✓ Key-only lines test passed"
\ No newline at end of file
diff --git a/action.yml b/action.yml
index f12181b..a9137fb 100644
--- a/action.yml
+++ b/action.yml
@@ -88,8 +88,9 @@ runs:
             escaped_key=$(echo "$key" | sed 's/[[\.*^$()+?{|]/\\&/g')
             
             # Check if key exists and track change type
-            if grep -q "^${escaped_key}=" "$FILE_PATH"; then
-              sed -i.bak "/^${escaped_key}=/d" "$FILE_PATH" && rm "${FILE_PATH}.bak"
+            # Match both "KEY=value" and key-only "KEY" lines (with optional trailing whitespace)
+            if grep -q "^${escaped_key}\(=\|[[:space:]]*$\)" "$FILE_PATH"; then
+              sed -i.bak "/^${escaped_key}\(=\|[[:space:]]*$\)/d" "$FILE_PATH" && rm "${FILE_PATH}.bak"
               echo "  ✏️  Updated: $key"
             else
               echo "  ➕ Added: $key"