When speakers duplicate drivers


What are the effects of duplicating drivers? I'm not talking about the so-called "half" driver where one does, say, both bass and midrange and the other does only bass, I'm talking about straight duplication.

What is the effect on sound? From a technical standpoint, what happens to the incoming signal when it gets split over duplicate drivers?
jimspov
If done correctly, increased power handling and greater maximum SPL. If not, phase cancellation, comb-filter effects, and erratic frequency response depending on listening angle relative to the speaker.
No multiple drivers covering the same frequency range for me!

Small differences (like phase), between drivers, show up as distortion.

I like 3 way, with 3 separate drivers.
Done correctly it can work quite well for more power handling. SPL and dynamics- especially bass output.  A good example would be two woofers handing off to a midrange driver at a low enough frequency (e.g. 400Hz) where distortion and phase issues are not audible.  A 2.5 way can accomplish the same thing.