quick question not worth a thread in legal discussion but someone might have been in this situation before:
my employer paid me late, then paid me again by accident. i didn't realise it was an accident and spent half the money on drugs. they now want me to write a cheque for the overpayment. do they have any legal means to make me? can i just not give it to them? i've already checked they can't just underpay me for my next payment....
Argh very similar thing happened to me, except they withdrew it themselves - I was on sick leave so wasn't sure if I was getting paid that month or not, saw I had been, spent it all on Christmas presents (and fun things no doubt) then realised on Christmas Eve that I was waaay over my overdraft limit

Sorry that isn't v helpful to your situation, am not sure about the legal standpoint.. are they amenable to paying in installments if you explain your situation? (I'd go for unforseen important expenses rather than drugs though hehe) It's so shit cos it's them who made the mistake - obv the money needs repaying but I think it's pretty mean to demand it straight away, they fucked up

Hope you get it sorted!