I consider that multiple uses of the spell would overlap, and that, as pointed by the rules, the stronger current effect would be applied... So, I made a simple table considering how 2 uses of mirror images in sequential turns would work and how attacks would affect them. The colors represent from which of the spells used is the currently seen image.
In this case, the colors represent each use of the spell "Mirror Image", that is, each time a spell slot and an action were spent to cast the spell (in the example, the spell was cast two times, the first time on turn 1, and the second time on turn 2).
The blue color represents the images created by the first use of the spell (I called it "A"), and the green color represents the images created by the second use of the spell (I called it "B").
The column "Total visible images the enemy sees in the current turn" represents the amount of mirror images that would be visible by people looking at the caster of the spell, and the colors of the cells in this column serves to point out if the images are derived from the first use of the spell (A), or from the second use of the spell (B).
To sum up, the main mechanic effects of two subsequent uses of the mirror images spell would be the following:
1st: Two turns must be spent to cast the spells.
2nd: Each group of images would have "an extra life" (meaning that each "image amount" would have to be "killed" twice in order to reduce the amount of images by 1).
The second effect is important because it represents a very interesting momentary buff for the evasion of the caster, considering the effects of having the higher number of duplicates for a longer period.
Here is a copy of part of the spell description that states the odds of redirecting an attack to one of the duplicates, based on the amount of duplicates still visible:
If you have three duplicates, you must roll a 6 or higher to change
the attack's target to a duplicate. With two duplicates, you must roll
an 8 or higher. With one duplicate, you must roll an 11 or higher.
Basically, each use of the spell grants the caster an extra turn of each group of duplicates or an extra attack in the same turn at each group (the group of 3 duplicates, the group of 2 duplicates, and the group of 1 duplicate).
It is also interesting to consider that maybe 3 or more subsequent uses of the spell might also be relevant, but only until the number reaches 10 (which is the moment that the first use of the spell would naturally expire, and the following uses of the spell would not overlap at more then 10 at a time, with one spell use expiring at each subsequent turn - what would make it necessary for the caster to spend his action to cast the spell again and again, on every turn, in order to keep the 10 "lifes" for his group of 3 duplicates). The number "10" is relevant, though, because at that turn, after casting the spell 10 times, the caster could recieve 10 attacks without lowering his amount of 3 visible images (which keeps the highest odds of redirecting attacks to one of the images even if all the 10 attacks manage to "kill" one image each). Consider this as an extremely situational scenery where the caster has 10 rounds to prepare himself to rush into an extremely dangerous area with lots of potential enemies (not to mention the huge amount of spell slots/scrolls or other forms of casting this spell that would be necessary). I'm not sure about making a table for this, but I think the idea is pretty funny.