Inspired by this question: Can you cast Mirror Image twice on successive turns and have 6 duplicates of yourself instead of 3?
Mirror Image (PHB, pg. 260) says:
Three illusory duplicates of yourself appear in your space. [...]
Each time a creature targets you with an attack during the spell's duration, roll a d20 to determine whether the attack instead targets one of your duplicates. [...]
A duplicate's AC equals 10 + your Dexterity modifier. In an attack hits a duplicate, the duplicate is destroyed. [...]
The PHB says in Chapter 10 under "Combining Magical Effects" (PHB pg. 205):
The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect - such as the highest bonus - from those castings applies while their durations overlap.
If you have two castings of Mirror Image on you, then you will only be under the effect of one of them at any given moment. However, which is considered the most potent as the one that is currently "active" starts losing duplicates?
For example, is it the case that the active one must lose all duplicates, then when you have none left, that spell ends and the second one "kicks in" and you have 3 again; or is it the case that as soon the first casting loses a duplicate, the second casting is considered active, so they effectively swap back and forth, so you end up with 3 duplicates, then 3, then 2, then 2, then 1, then 1, before finally having none when both spells have ended?
And would the answer change depending on whether one of them was cast with a higher spell slot than the other, since that's the other way I'd think to determine potency?