Think how bad some dude feels after he knocks up his wifey and she has fucking triplets. That's like... a '' You're not gonna get laid for 6 fucking years because your jizz is super effective'' kind of thing.
its most likely not weed lots of soils contain non desired seeds that usually only sprout indoors.....i noticed youre in the UK so we probably dont have the same soils, but are you using a storebought soil?
is it a big name brand or their own mix? i definitely think its a weed seed (the bad kind not cannabis) but regardless, growing two plants in one small container is bad because the roots will tangle and fuck up your yield. i suppose you could remove it and let it grow seperately for giggles
Tomorite is the brand. I'll leave it a day or so and see what it is. If its just a general weed then i'll rip it out.