Don't know the answer, but (and I can't see the scan clearly) I assume that unless one is an overprint of the other, then this is an error. As £1.00 is twice 10/- (ten shillings) then there should obviously not be both on one chip.

Maybe, as is the case in the UK back in 1970, the shilling is dropped as a monetary unit, then maybe a casino has attempted to use the chips as a different denomination rather than just scrap the chip?

Jason