[Help]Adobe Audition copy & paste

[Donnje]

Hi,
I cannot copy part audio in another track
Eg. I have a multitrack with movie + track1 + track2, if I copy a portion of the second track and paste it in the first track, this is not copied but you replace the track 1 for the same time
This problem isn't there with a single track without using multitrack
Is there any setting to change or am I wrong something?
Thanks