a couple of things:
1. the amount is presumably quite small (1 month increment) so is it really worth keeping?
2. if the company finds out and you resist repaying (if even legal), they'll get it back somehow - eg. a 3% payrise next year instead of 4%, or reduced (if any) bonus.
3. if they find out and you refuse to repay, then your future at this company is over, you just won't realise it.
4. how financially savvy do they think you are? if you have previously been underpayed, how quick were you to flag it? if they find out without you notifying them, someone will figure out that there is a high probability you knew but said nothing, again game over.
do the sensible thing, flag it now, get them to deduct it from next pay run and everyone's happy.