120 fps double playback time

Hello,

may someone can explain me..

I tried Premiere CS6 while looking for a Programm to work on my GoPro Videos.

I imported a 720p 120 fps Video and is was playing "normal".

BUT, then i noticed, that the video, what should be about 15 minutes long, has double length about 30 minutes in the timeline and also when i export it with 60 fps.

When i play the source file it is 15 minutes long. In other Video editing tools, it is 15 minutes.. only in Premiere it get 30 minutes.

so why? and how can i change that to the original time?

i tried with "change speed" but then, audio doesn't sound original or gets asyncronous.

thanks

Christian