diff --git a/package.json b/package.json
index 88efade..2996961 100644
--- a/package.json
+++ b/package.json
@@ -18,8 +18,9 @@
     "test:nested": "cd test/nested && rm -rf output && rollup -c && cmp output/output.js expected.js && cmp output/output.css expected.css && cd ../..",
     "test:empty": "cd test/empty && rm -rf output && rollup -c && cmp output/output.js expected.js && cmp output/output.css expected.css && cd ../..",
     "test:simple": "cd test/simple && rm -rf output && rollup -c && cmp output/output.js expected.js && cmp output/output.css expected.css && cd ../..",
+    "test:unique": "cd test/unique && rm -rf output && rollup -c && cmp output/main.js expected.js && cmp output/output.css expected.css && cd ../..",
     "test:win:simple": "cd .\\test\\simple && del -f output.* && rollup -c && cd .. && ECHO n|comp simple\\output.js expected.js && ECHO n|comp simple\\output.css simple\\expected.css && cd ..",
-    "test": "npm run test:simple && npm run test:nested && npm run test:empty && npm run test:circular",
+    "test": "npm run test:simple && npm run test:nested && npm run test:empty && npm run test:circular && npm run test:unique",
     "test:win": "npm run test:win:simple",
     "lint": "prettier rollup.config.js src/**",
     "prepare": "npm run build",
diff --git a/src/index.mjs b/src/index.mjs
index 6fba44f..a82c4c0 100644
--- a/src/index.mjs
+++ b/src/index.mjs
@@ -59,17 +59,17 @@ export default function css(options = {}) {
       return ''
     },
     generateBundle(opts, bundle) {
-      const ids = []
+      const ids = new Set()
 
       // Determine import order of files
       for (const file in bundle) {
         const root = bundle[file].facadeModuleId
         const modules = getCSSModules(root, this.getModuleInfo)
-        ids.push(...Array.from(modules))
+        modules.forEach(id => ids.add(id))
       }
 
       // Combine all stylesheets, respecting import order
-      const css = ids.map(id => styles[id]).join('\n')
+      const css = Array.from(ids).map(id => styles[id]).join('\n')
 
       // Emit styles through callback
       if (typeof options.output === 'function') {
diff --git a/test/unique/dependency-a.js b/test/unique/dependency-a.js
new file mode 100644
index 0000000..2bb00d7
--- /dev/null
+++ b/test/unique/dependency-a.js
@@ -0,0 +1,3 @@
+import './input.css'
+
+console.log('dependency-a');
diff --git a/test/unique/expected.css b/test/unique/expected.css
new file mode 100644
index 0000000..0ce1a39
--- /dev/null
+++ b/test/unique/expected.css
@@ -0,0 +1,4 @@
+.rollup {
+  color: green;
+  user-select: none;
+}
diff --git a/test/unique/expected.js b/test/unique/expected.js
new file mode 100644
index 0000000..5a2afd1
--- /dev/null
+++ b/test/unique/expected.js
@@ -0,0 +1 @@
+import('./dependency-a.js');
diff --git a/test/unique/input.css b/test/unique/input.css
new file mode 100644
index 0000000..8963c8e
--- /dev/null
+++ b/test/unique/input.css
@@ -0,0 +1,4 @@
+.rollup {
+  color: green;
+  user-select: none;
+}
\ No newline at end of file
diff --git a/test/unique/main.js b/test/unique/main.js
new file mode 100644
index 0000000..293091d
--- /dev/null
+++ b/test/unique/main.js
@@ -0,0 +1,2 @@
+import './input.css'
+import('./dependency-a.js')
diff --git a/test/unique/output/dependency-a.js b/test/unique/output/dependency-a.js
new file mode 100644
index 0000000..3ddc4e6
--- /dev/null
+++ b/test/unique/output/dependency-a.js
@@ -0,0 +1 @@
+console.log('dependency-a');
diff --git a/test/unique/output/main.js b/test/unique/output/main.js
new file mode 100644
index 0000000..5a2afd1
--- /dev/null
+++ b/test/unique/output/main.js
@@ -0,0 +1 @@
+import('./dependency-a.js');
diff --git a/test/unique/rollup.config.mjs b/test/unique/rollup.config.mjs
new file mode 100644
index 0000000..bcb1aa6
--- /dev/null
+++ b/test/unique/rollup.config.mjs
@@ -0,0 +1,11 @@
+import css from '../../src/index.mjs'
+
+export default {
+  input: 'main.js',
+  output: {
+    chunkFileNames: '[name].js',
+    dir: 'output',
+    format: 'esm'
+  },
+  plugins: [css({ output: 'output.css' })]
+}