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?
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I have such a speaker which I'm listening to right now. Paradyme Phantom V4. 2-way floorstander w/ two 6 1/2" drivers. I always preferred 2-ways for some reason. This loudspeaker has fuller sound which I believe goes down to 40hz. They are still 8 ohm spks. I would have thought the impedance would be lower w/the extra driver.
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.
Not a fan of driver arrays for home audio.   
Here’s one with 12 of the same woofers and 9 of the same tweeters in one speaker cabinet.


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.  
Google "D'Appolito configuration" and you'll get one argument for this approach.
My speaker have this and sound WONDERFUL!!!!!!
I have Monitor Audio PL-500's and they are the best speakers that I've ever owned.
Three 6.5 inch drivers handing off(1700Hz) to a 5.75 inch tractrix horn with a 1.25 inch compression titanium driver.  Very smooth transition with no cancellation or phase issues that are noticeable.

Years ago I owned Allison one and ADS 810 speakers. Both had duplicate woofers. The Allison also had duplicate mid and tweeters . They were both oustanding  !

Using two woofer to cover the same frequency range is neither inherently good nor bad, it just depends on what the designer is trying to do.

Compared to using a single woofer, using two of that woofer to cover the same frequency range lowers the distortion and power compression for a given SPL because less excursion is required along with less power, but you need twice the box volume. Assuming both woofers are facing the same direction, the radiation pattern will be considerably narrower in the plane where their center-to-center spacing is the greatest (for an MTM, that would be the vertical plane),. Whether or not this is a good thing or a bad thing depends on the rest of the system design.

Recently I designed a speaker where one of the goals was to reduce the amount of energy in early reflections via fairly aggressive radiation pattern control. The best constant-directivity waveguide I could find for this application had a pattern 90 degrees horizontally by about 50 degrees vertically. Using two midwoofers covering the same frequency range was the best way to get the directional control I wanted along with good pattern-matching in the crossover region in both the horizontal and vertical planes.

If my priorities had been different, I might well have used just one midwoofer.