noun
1. Video game. A term for a multiplayer match or simulation in which both opposing sides are the same character, characters, or group of characters.
EX. Mirror matches have, in League of Legends, been only a 1 on 1 match-up because in blind pick you can only have one of a single character on each team and in draft pick matches things become even more strict with only one copy of a character between both team. This meant that, while some players have wanted to see what would happen in a 5 V 5 match between all Ashes, Luxes, Blitzcranks, etc, such dream matches were impossible...
...until the other day when some people started to discover they could create buggy, difficult, but functional matches in custom games that allow you to pull of this feat. Check out the response from Riot, HERE. It seems we've got a new mode to look forward to in the near future but, for now, I'll have to see if I can get into a Teemo match the tricky way.
You should also check out Dunkey's experiences in these "dupe matches" in the link below.
