[enh] engine shortcut validation

searx/engines/__init__.py

         if engine_attr.startswith('_'):
             continue
         if getattr(engine, engine_attr) is None:
-            print '[E] Engine config error: Missing attribute "{0}.{1}"'.format(engine.name, engine_attr)  # noqa
+            print('[E] Engine config error: Missing attribute "{0}.{1}"'\
+                  .format(engine.name, engine_attr))
             sys.exit(1)
 
     engine.stats = {
 
     if engine.shortcut:
         # TODO check duplications
+        if engine.shortcut in engine_shortcuts:
+            print('[E] Engine config error: ambigious shortcut: {0}'\
+                  .format(engine.shortcut))
+            sys.exit(1)
         engine_shortcuts[engine.shortcut] = engine.name
     return engine