Is the sentence "Didn't you have a meeting to attend to" grammatically correct?
Yes, it is completely correct. It is the equivalent of
You had a meeting to give your attention to, did you not?
I have some urgent business to attend to.
I think your confusion might stem form your understanding of to attend
- to be present at (an event, meeting, etc)
- (when: intr, foll by to) to give care; minister
- (when: intr, foll by to) to pay attention; listen
There is no "to" built into the definition of attend; in usage, attend to is very common.