diff --git a/lib/compiler/index.js b/lib/compiler/index.js
index 40c05f3..d3edda9 100644
--- a/lib/compiler/index.js
+++ b/lib/compiler/index.js
@@ -19,16 +19,17 @@ var compiler = {
    */
   passes: {
     check: {
-      reportMissingRules:  require("./passes/report-missing-rules"),
-      reportLeftRecursion: require("./passes/report-left-recursion"),
-      reportInfiniteLoops: require("./passes/report-infinite-loops")
+      reportMissingRules:   require("./passes/report-missing-rules"),
+      reportDuplicateRules: require("./passes/report-duplicate-rules"),
+      reportLeftRecursion:  require("./passes/report-left-recursion"),
+      reportInfiniteLoops:  require("./passes/report-infinite-loops")
     },
     transform: {
-      removeProxyRules:    require("./passes/remove-proxy-rules")
+      removeProxyRules:     require("./passes/remove-proxy-rules")
     },
     generate: {
-      generateBytecode:    require("./passes/generate-bytecode"),
-      generateJS:          require("./passes/generate-js")
+      generateBytecode:     require("./passes/generate-bytecode"),
+      generateJS:           require("./passes/generate-js")
     }
   },
 
diff --git a/lib/compiler/passes/report-duplicate-rules.js b/lib/compiler/passes/report-duplicate-rules.js
new file mode 100644
index 0000000..44cfa27
--- /dev/null
+++ b/lib/compiler/passes/report-duplicate-rules.js
@@ -0,0 +1,26 @@
+"use strict";
+
+var GrammarError = require("../../grammar-error"),
+    visitor      = require("../visitor");
+
+/* Checks that each rule is defined only once. */
+function reportDuplicateRules(ast) {
+  var rules = {};
+
+  var check = visitor.build({
+    rule: function(node) {
+      if (rules.hasOwnProperty(node.name)) {
+        throw new GrammarError(
+          "Rule \"" + node.name + "\" is already defined.",
+          node.location
+        );
+      }
+
+      rules[node.name] = true;
+    }
+  });
+
+  check(ast);
+}
+
+module.exports = reportDuplicateRules;
diff --git a/package.json b/package.json
index 3f5d20d..032326f 100644
--- a/package.json
+++ b/package.json
@@ -31,6 +31,7 @@
     "lib/compiler/passes/report-left-recursion.js",
     "lib/compiler/passes/report-infinite-loops.js",
     "lib/compiler/passes/report-missing-rules.js",
+    "lib/compiler/passes/report-duplicate-rules.js",
     "lib/grammar-error.js",
     "lib/parser.js",
     "lib/peg.js",
diff --git a/spec/index.html b/spec/index.html
index db65963..d59581b 100644
--- a/spec/index.html
+++ b/spec/index.html
@@ -10,6 +10,7 @@
     <script src="unit/parser.spec.js"></script>
     <script src="unit/compiler/passes/helpers.js"></script>
     <script src="unit/compiler/passes/report-missing-rules.spec.js"></script>
+    <script src="unit/compiler/passes/report-duplicate-rules.spec.js"></script>
     <script src="unit/compiler/passes/report-left-recursion.spec.js"></script>
     <script src="unit/compiler/passes/report-infinite-loops.spec.js"></script>
     <script src="unit/compiler/passes/remove-proxy-rules.spec.js"></script>
diff --git a/spec/unit/compiler/passes/report-duplicate-rules.spec.js b/spec/unit/compiler/passes/report-duplicate-rules.spec.js
new file mode 100644
index 0000000..a74554e
--- /dev/null
+++ b/spec/unit/compiler/passes/report-duplicate-rules.spec.js
@@ -0,0 +1,20 @@
+/* global peg */
+
+"use strict";
+
+describe("compiler pass |reportDuplicateRules|", function() {
+  var pass = peg.compiler.passes.check.reportDuplicateRules;
+
+  it("reports duplicate rules", function() {
+    expect(pass).toReportError([
+      'start = "a"',
+      'start = "b"'
+    ].join('\n'), {
+      message:  'Rule "start" is already defined.',
+      location: {
+        start: { offset: 12, line: 2, column:  1 },
+        end:   { offset: 23, line: 2, column: 12 }
+      }
+    });
+  });
+});