Prefererences>Recording has the Sound-on-Sound (blend) option enabled.

I have a MIDI track with several take lanes.
If I record-enable one of the lanes, with existing clips in it, and start recording MIDI notes, SONAR erases existing notes from the clips.
I have to use a second, empty, take lane in order to perform sound-on-sound midi recording.

Is this a bug? 

That sounds like a bug. I'd expect a new MIDI clip to get added on top of the existing one in that lane if Sound-on-Sound is on. 

Does it work the same for audio?

Funky - 

For audio tracks, it's different, but still "wrong", I think.

If I overlap the original clip, it gets slip edited so it appears that the new clip replaces the portion of the old one. In this case I can get the audio data back by moving clips around and undoing the slip-editing.

However, If I completely overwrite the original clip, it gets deleted - or at least, I can't get to it in the project any more. I guess the audio data is in the project folder, still.

This doesn't feel right. And it is quite different from how Layers worked in X1. 

I'm beginning to understand what BladeTragic was on about in the other thread.

there is a separate thread about midi punch in recording.. this behaviour seems to be the same on both midi and audio overwriting lanes.. the first overwrite will delete the original take...

i would think this would be a bug.. if this is intended operation then we need a feature request NOT to delete prior recorded material