message sensor

hi guys
My problem is that whenever i use a message sensor things happen twice. For example I’m using DupliGroups to add enemies to my stage and they are always added twice even with tap active. what can I do about this?

Is this with Python, or just logic?

You might want to post a blend file.

I had the same problem a while ago.
My python script received two messages, one of them with the correct value and one empty message.
I think I solved the problem by checking if the body is empty.