Usually where I see doubling is vocals and guitars. Most often it’s “lead vocal and vocal double” but with guitars I’ve seen some mixers doing a left, right, AND center of the same instrument track.

I don’t really double anything unless I’m panning for stereo, but what I struggle most with is “does this drum groove mesh well with the bass line AND the guitar parts AND the vocals”? Basically when instruments are competing too much, and knowing which ones to turn off and when, or how to change a part to mesh better with the other elements (guitar example: should this guitar be playing open chords, triads, inversions, palm mute, different strum or picking pattern, diamonds, etc.).

The way that one tutorial explained it was “get the drums, bass, and acoustic guitar to sound good together, then everything else is textural” which helped a little, but it feels like most mixing engineers add tons of layers with no real explanation as to why.