Would you classify common time © as simple duple or simple quadruple time? Or would it depend on the speed/feel of the piece?

Duple.

C without a line through it is simple quadruple (4/4).
With a line though is simple duple (2/2).

common time, i.e. 4/4, is by definition quadruple time. however, the term "duple" is often extended to include quadruple.

sometimes you'll see a tune that is notated in common time but when played is clearly cut time; this is quite common in traditional music (for example, reels are frequently notated in common time, but you'd be hard pressed to find four beats in a bar). in this case, the tune is in cut time, therefore duple, it's just been notated wrongly.

aha, that makes sense in the context I was puzzling over.

Thanks guys!