Tracks are merging

I have a problem when I use multitrack recording. No problem when I record the first track, but when I start overdubbing, the first track is merging into the second track with the new recording. I can’t find any solutions on any troubleshooting-pages… What am I doing wrong?

Ensure that you select the actual device that you are recording from (such as a microphone or line in) in the Device Toolbar and not “Stereo Mix” or “Microsoft Sound Mapper”.
If you are recording with a microphone, use headphones and not speakers or the microphone will pick up sound from the speakers.
If you are using a mixing desk, check the signal routing.