diff --git a/src/createUniqueIdentifiers.js b/src/createUniqueIdentifiers.js
new file mode 100644
index 00000000..151930c1
--- /dev/null
+++ b/src/createUniqueIdentifiers.js
@@ -0,0 +1,16 @@
+
+module.exports = function createUniqueIdentifiers(identifiers) {
+  const dupeCount = {};
+
+  return identifiers.map((identifier) => {
+    if (typeof dupeCount[identifier] !== 'undefined') {
+      dupeCount[identifier] += 1;
+    } else {
+      dupeCount[identifier] = 0;
+    }
+
+    return dupeCount[identifier] === 0
+      ? `${identifier}`
+      : `${identifier}_${dupeCount[identifier]}`;
+  });
+};
diff --git a/src/index.js b/src/index.js
index 30bc49b5..f41eafcd 100644
--- a/src/index.js
+++ b/src/index.js
@@ -17,12 +17,20 @@ module.exports.pitch = function pitch(remainingRequest) {
   }
 
   const insertCssPath = path.join(__dirname, './insertCss.js');
+  const createUniqueIdentifiersPath = path.join(__dirname, './createUniqueIdentifiers.js');
   let output = `
     var content = require(${stringifyRequest(this, `!!${remainingRequest}`)});
     var insertCss = require(${stringifyRequest(this, `!${insertCssPath}`)});
+    var createUniqueIdentifiers =
+      require(${stringifyRequest(this, `!${createUniqueIdentifiersPath}`)});
 
     if (typeof content === 'string') {
       content = [[module.id, content, '']];
+    } else {
+      var identifiers = content.map(x => x[0]);
+      createUniqueIdentifiers(identifiers).map((identifier, index) => {
+        content[index][0] = identifier;
+      })
     }
 
     module.exports = content.locals || {};
diff --git a/src/insertCss.js b/src/insertCss.js
index 7d6c1414..37fd0c91 100644
--- a/src/insertCss.js
+++ b/src/insertCss.js
@@ -8,7 +8,7 @@
  */
 
 const prefix = 's';
-const inserted = {};
+let inserted = {};
 
 // Base64 encoding and decoding - The "Unicode Problem"
 // https://developer.mozilla.org/en-US/docs/Web/API/WindowBase64/Base64_encoding_and_decoding#The_Unicode_Problem
@@ -49,8 +49,7 @@ function insertCss(styles, options) {
 
   const ids = [];
   for (let i = 0; i < styles.length; i++) {
-    const [moduleId, css, media, sourceMap] = styles[i];
-    const id = `${moduleId}-${i}`;
+    const [id, css, media, sourceMap] = styles[i];
 
     ids.push(id);
 
@@ -103,4 +102,8 @@ function insertCss(styles, options) {
   return removeCss.bind(null, ids);
 }
 
+insertCss._clearCache = () => {
+  inserted = {};
+};
+
 module.exports = insertCss;
diff --git a/test/createUniqueIdentifiersSpec.js b/test/createUniqueIdentifiersSpec.js
new file mode 100644
index 00000000..d298f415
--- /dev/null
+++ b/test/createUniqueIdentifiersSpec.js
@@ -0,0 +1,60 @@
+/**
+ * Isomorphic CSS style loader for Webpack
+ *
+ * Copyright © 2015-2016 Kriasoft, LLC. All rights reserved.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE.txt file in the root directory of this source tree.
+ */
+
+import { expect } from 'chai';
+import createUniqueIdentifiers from '../src/createUniqueIdentifiers';
+
+const { describe, it } = global;
+
+describe('createUniqueIdentifiers', () => {
+  describe('when all identifiers are unique', () => {
+    const identifiers = ['1', '23', '13'];
+
+    it('retains all identifiers', () => {
+      expect(createUniqueIdentifiers(identifiers)).to.deep.equal(identifiers);
+    });
+  });
+
+  describe('when some of the identifiers are non-unique', () => {
+    const identifiers = ['0', '12', '12', '23'];
+
+    it('makes non-unique identifiers unique', () => {
+      const uniqueIdentifiers = createUniqueIdentifiers(identifiers);
+      expect(uniqueIdentifiers[1]).to.not.equal(uniqueIdentifiers[2]);
+    });
+
+    it('retains unique identifiers', () => {
+      const uniqueIdentifiers = createUniqueIdentifiers(identifiers);
+      expect(uniqueIdentifiers[0]).to.equal(identifiers[0]);
+      expect(uniqueIdentifiers[3]).to.equal(identifiers[3]);
+    });
+  });
+
+
+  describe('when there are multiple groups of non-unique identifiers', () => {
+    const identifiers = ['12', '14', '12', '4', '800', '800', '801', '12'];
+
+    it('makes non-unique identifiers unique', () => {
+      const uniqueIdentifiers = createUniqueIdentifiers(identifiers);
+
+      expect(uniqueIdentifiers[0]).to.not.equal(uniqueIdentifiers[2]);
+      expect(uniqueIdentifiers[0]).to.not.equal(uniqueIdentifiers[7]);
+      expect(uniqueIdentifiers[2]).to.not.equal(uniqueIdentifiers[7]);
+
+      expect(uniqueIdentifiers[4]).to.not.equal(uniqueIdentifiers[5]);
+    });
+
+    it('retains unique identifiers', () => {
+      const uniqueIdentifiers = createUniqueIdentifiers(identifiers);
+      expect(uniqueIdentifiers[1]).to.equal(identifiers[1]);
+      expect(uniqueIdentifiers[3]).to.equal(identifiers[3]);
+      expect(uniqueIdentifiers[6]).to.equal(identifiers[6]);
+    });
+  });
+});
diff --git a/test/insertCssSpec.js b/test/insertCssSpec.js
index 58cc61a1..a1d9c820 100644
--- a/test/insertCssSpec.js
+++ b/test/insertCssSpec.js
@@ -8,37 +8,69 @@
  */
 
 import jsdom from 'jsdom';
-import { describe, it } from 'mocha';
 import { expect } from 'chai';
 import insertCss from '../src/insertCss';
 
+const { describe, it, beforeEach } = global;
+
 global.document = jsdom.jsdom('<!doctype html><html><body></body></html>');
 global.window = document.parentWindow;
 
+const css1 = 'body { color: red; }';
+const css2 = 'body { color: blue; }';
+
+function getStyleTags() {
+  return global.document.getElementsByTagName('style');
+}
+
 describe('insertCss(styles, options)', () => {
-  it('Should insert and remove <style> element', () => {
-    const css = 'body { color: red; }';
-    const removeCss = insertCss([[1, css]]);
-    let style = global.document.getElementById('s1-0');
-    expect(style).to.be.ok;
-    expect(style.textContent).to.be.equal(css);
-    expect(removeCss).to.be.func;
-    removeCss();
-    style = global.document.getElementById('s1-0');
-    expect(style).to.be.null;
+  beforeEach(() => {
+    insertCss._clearCache();
+    const styles = global.document.getElementsByTagName('style');
+    for (const style of styles) {
+      style.parentElement.removeChild(style);
+    }
+  });
+
+  it('inserts a style element', () => {
+    insertCss([[1, css1]]);
+    const styleTags = getStyleTags();
+    expect(styleTags[0].textContent).to.equal(css1);
   });
 
-  it('Should insert and remove multiple <style> elements for a single module', () => {
-    const css1 = 'body { color: red; }';
-    const css2 = 'body { color: blue; }';
-    const removeCss = insertCss([[1, css1], [1, css2]]);
-    let style = global.document.getElementsByTagName('style');
-    expect(style.length).to.be.equal(2);
-    expect(style[0].textContent).to.be.equal(css1);
-    expect(style[1].textContent).to.be.equal(css2);
-    expect(removeCss).to.be.func;
+  it('returns a function that removes the style element', () => {
+    const removeCss = insertCss([[1, css1]]);
+    expect(removeCss).to.be.a('function');
     removeCss();
-    style = global.document.getElementsByTagName('style');
-    expect(style.length).to.be.equal(0);
+    const styleTags = getStyleTags();
+    expect(styleTags.length).to.equal(0);
+  });
+
+  describe('when a module is added a second time', () => {
+    it('does nothing', () => {
+      insertCss([[1, css1]]);
+      insertCss([[1, css2]]);
+      const styleTags = getStyleTags();
+      expect(styleTags.length).to.equal(1);
+      expect(styleTags[0].textContent).to.equal(css1);
+    });
+
+    describe('and options.replace is set to true', () => {
+      it('replaces the first module', () => {
+        insertCss([[1, css1]]);
+        insertCss([[1, css2]], { replace: true });
+        const styleTags = getStyleTags();
+        expect(styleTags.length).to.equal(1);
+        expect(styleTags[0].textContent).to.equal(css2);
+      });
+    });
+  });
+
+  describe('when a module is imported from multiple places', () => {
+    it('only inserts it once', () => {
+      insertCss([[2, css1]]);
+      insertCss([[1, css1], [2, css2]]);
+      expect(getStyleTags().length).to.equal(2);
+    });
   });
 });
diff --git a/test/withStylesSpec.js b/test/withStylesSpec.js
index 4cc6a467..53c8c9fd 100644
--- a/test/withStylesSpec.js
+++ b/test/withStylesSpec.js
@@ -9,11 +9,11 @@
 
 /* eslint-disable react/prefer-stateless-function */
 
-import { describe, it } from 'mocha';
 import { expect } from 'chai';
 import React, { createClass, Component } from 'react';
 import withStyles from '../src/withStyles';
 
+const { describe, it } = global;
 
 describe('withStyles(ComposedComponent, ...styles)', () => {
   class Passthrough extends Component {