Some may not know, but the first piece of Sonic music to be reused is "You Can Do Anything (Toot Toot Sonic Warrior)" from the Japanese/European Sonic CD.
Here is the original track.

Here's another one. This one surprised me.

What's so bad about Sonic reusing it's own music? Mario does it ALL THE TIME.

Uncommon? I thought they did that a lot in Sonic Adventure (1). I was pretty sure that they did it once per stage a lot. It was remixed a little, but they definitely used some 3D Blast music in it.

Actually, it was only three times.

1. Second half of Emerald Coast (Remix of Spring Stadium from 3D Blast)
2. Last third of Windy Valley (Remix of Green Grove from 3D Blast)
3. First Half of Twinkle Park (Remix of Puppet Panic from 3D Blast)

If I'm wrong, then please tell me.

Here's another one: