Fail when using same command and alias

index.js

@@ -851,6 +851,7 @@ Command.prototype.description = function(str) {
 
 Command.prototype.alias = function(alias) {
   if (0 == arguments.length) return this._alias;
+  if (alias === this._name) throw new Error('Command alias can\'t be the same as its name');
   this._alias = alias;
   return this;
 };

test/test.command.failOnSameAlias.js

@@ -0,0 +1,14 @@
+var program = require('../')
+  , should = require('should');
+
+var error;
+
+try {
+  program
+    .command('fail')
+    .alias('fail');
+} catch (e) {
+  error = e;
+}
+
+error.should.deepEqual(new Error('Command alias can\'t be the same as its name'));