Nice try

It depends if the machine dispensed a receipt that showed that it gave out 100 gbp and actually gave 200. The receipt I think will have to be the basis.

It's a simple error.  Some branch twonk has stuck £20 notes in the £10 cassette.  In some cassettes this is easy; but in some it is more difficult, involving large numbers of branch staff sitting on the device in an attempt to make all of the closing parts close.  Sometimes they notice, sometimes they don't. 

Question is, is it really worth chasing up the lucky punters who have just doubled their money?  In most cases, it isn't, so good luck to them.  Now if the ATM dispenses several thousand pounds too much, like the Halifax Philips 6601 ATMs did in 1988, that's a different story, and off go the assistant branch dogsboddies to try and retrieve it.  For the sake of a monkey or a pony, though, they will just shut the stable door after the horse has bolted.