Error messages now do not contain duplicities.

lib/runtime.js

@@ -93,7 +93,10 @@ PEG.Parser.prototype = {
       this._rightmostMatchFailuresPos = this._pos;
       this._rightmostMatchFailuresExpected = [];
     }
-    this._rightmostMatchFailuresExpected.push(failure);
+
+    if (this._rightmostMatchFailuresExpected.indexOf(failure) === -1) {
+      this._rightmostMatchFailuresExpected.push(failure);
+    }
   },
 
   /*

test/compiler-test.js

@@ -456,6 +456,13 @@ test("error messages", function() {
     "something",
     'Expected end of input but "s" found.'
   );
+
+  var duplicateErrorParser = PEG.buildParser('start: "a" / "a"');
+  doesNotParseWithMessage(
+    duplicateErrorParser,
+    "",
+    'Expected "a" but end of input found.'
+  );
 });
 
 test("error positions", function() {