The Mikasa Glitch is a glitch that has been encountered by several loopers over the history of the loops.
The Mikasa Glitch had been encountered several times before being formally named; with several known events being Pikachu encountering an additional version of himself during a Super Smash Loops while Dr. Light once was split in two as both Edison and Telsa, however this was little information on this glitch known due to no one really studying it in detail.
The glitch was studied in detail, and given a name, by Armin Arlert, who studied the glitch in detail during a loop where the glitch caused 49 looping Mikasa Ackerman's to run around at once. His research was later compiled into several research journals, and made avaliable to the other loopers.
The glitch can also be imitated to a physical extent by the MTG spell Splinter Twin. This form does not have memories or pockets, but it has all other powers. These clones degrade over time, however.
When this glitch is in effect, a looper will find at least one other version of themselves active in the loop at once. As oppose to a Mini Me loop, this individual would have all the memories and powers of the looper, their own duplicate of the looper's subspace pocket, and would believe himself to be the real one. While there is generally only one, in the event with Mikasa there were 49 Mikasas found in total. They also can come in different genders than the original, though this has yet to be studied in full detail.
According to Armin's research, loopers who are affected by this glitch become rather confused and are often prone to conflict with their duplicates, either due to the conflicts caused by the duplicate and original looper arguing with one another over who is real (This was seen in Mikasa when one suggested that the others' scarfs were fakes) or because they are prone to violence in general (In the conflict between Anakin and 'Annie' Skywalker)
When the loop ends, the looper in question gains all the memories the clone had of the loop, similar to if one had a Shadow Clone be disspelled. In effect, this means that the 'clone' is not a 'fake' version of the looper and is, in fact, every bit as real as the 'original'. As such, any designation of 'clone' or 'original' in regards to those under the effects of the glitch is little more than semantics.
What to do in this situationEdit
The best thing to do in the event of this glitch is to try to make peace with your duplicate. This can be difficult, but it can be done; as they share the original's emotions they will react to the same things you would. For example, the Mikasa's stopped trying to kill each other when they realized they were terrifying Eren Yaeger, the person that they cared about.
While dueling your clone to the death could in theory get rid of him/her, this is not suggested. The duplicate shares all your powers and thoughts, making such a conflict a long and unguaranteed struggle. At the same time, it could easily destroy a loop; for instance when Anakin and his female clone fought, they ended up destroying the Mid and Outer Rim planets before triggering a spiral nemesis. And that's not even mentioning the fact that next Loop you'd remember dying at your own hand anyway...