Welcome to the forum!
You give no details whatsoever (hardware specs, track routing, recording method etc) to start solving the problem.
Anyways, the most usual cause of the issue you describe is that the soundcard has a feature called
"Stereo Mix" or similar enabled (the naming may vary, I don't remember the usual alternatives).
Are you talking about recording audio or soft synths? I assume it's audio.
If MIDI is involved, make sure every instrument uses its own MIDI channel.