During online gameplay on the Xbox 360 game 'Halo' - variations of which can be researched separately - an odd phenomenon often occurs in which both yourself and your enemy kill eachother in the same instance. It is a quite unique feature of the alien-themed battle-game, and is usually both hugely frustrating and hard to explain, in equal measures.
Therefore, lo and behold, 'the doubler' was coined. This phrase makes it easy to tell one's teammate of your unfortunate end, and marvel at how clearly you meleed first.
Nigel 'whats up'
James 'only the bloody doubler again. and i clearly shot him in the face more than he got me'
Nigel 'the doubler, huh? typical. teabag him next time if you get the chance'