I would rule that the original and the duplicates all have the effect of faerie fire visible. Faerie fire would outline all the duplicates and give advantage on attack rolls regardless. However while the spell does say

A duplicate can be destroyed only by an attack that hits it. It ignores all other damage and effects.

Mirror Image also says

Until the spell ends, the duplicates move with you and mimic your actions, shifting position so it's impossible to track which image is real.

I think that the duplicates would not be affected by faerie fire directly BUT seeing as the original is affected, they would alter their appearance to match that of the original thus conferring advantage to attacks made on them as the advantage is a result of faerie fire's outline making it easy to see the affected creature. (Otherwise why would the spell have to specify "if the attacker can see it"?)

The spell with the most images would become active (all else being equal)

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.

This is going to come down to how the DM rules how to measure potency as the game doesn't tell us a definitive answer for what should count. It seems reasonable to say that, all other things being equal, that the spell with the most images would be considered the most potent effect at the time.

How this would play out in practice would be that whenever a spell's number of images is reduced below that of the other spell, the other spell would become active. When both spells have equal images (and everything else) it really doesn't matter which spell is active done they are identical. The alternating behavior would continue until one spell loses all of its images.

What if all else is not equal?

The above assumes that the spells are identical in every respect (duration, AC, etc.) if there is any other discrepancies the DM will have to choose how they define potency in more detail. For example, if one mirror image was cast a round later than the other one it would be completely reasonable to rule that the one with the longer duration remaining would be the more potent spell and that it would override the spell for the entire duration. This would also be the case if one of the spells was cast at a higher level than the other (and everything else was the same).