If your bank or credit card account shows an unexpected £20 or £40 debit for TfL then it’s likely that you were involved in a systems issue in February. Around 750 Oyster cards were auto topped up on Wednesday 19th February but when TfL tried to collect the funds the next day there was a problem. The issue has now been rectified and TfL re-processed the transactions on 1st May.
If this scenario makes sense then you don’t need to do anything, but you are welcome to contact the helpdesk with your Oyster card number and they will confirm whether or not you were involved.