However, if it's in your account and you "genuinely" have no idea it was a mistake, you can always put it in a high interest savings account, so it's still there to give back, but you'll have earned your few pounds for it using your account. Fair!
Contact the Bank and complain bitterly about them not being able to maintain your account correctly. Demand to know how this happened. They will fix it, and might give you some amount as an apology...