To which ....... to

That's the song we danced to.

The final to is superfluous, so i suppose it's bad grammar.

All you need write is: That's the song to which we danced.

That's the restaurant at which we ate.

That's the church in which we were married.

Get it?

Cool.
Yeah, it's terse and exact.

Eliminate one "to" and your sentence will be fine. For example, you can either write "That's the song to which we danced" or "That's the song we danced to".

I suppose it's not correct.
You can say ''That's the song to which we danced. '' or '' That's the song which we danced to. ''