FaceTime plays sound double at receivers side

i have the following problem.

when i call someone or someone calls me, the other participant hears himself twice.

in the first moment i thought that i was just the problem with the others persons mac, but then i made several other facetime calls with my macbook pro 13 (june 2012) and all the other people complained that they can hear themselves again, like an echo.

when i call from my iphone, everthing is fine. the receiver hears me perfectly and no problem appears.

this problem started like one week ago. the apple support could not help me yet and i didnt find a similar problem on the web neither. i didnt install any new apllication the last weeks or i didnt do any change in my computer.

i already tried to delete the facetime app but it says that im not able to delete it because its part of the OS. (mavericks)

i hope anybody has an idea to fix this

thank you!

regards

edit: skype works fine