I've never heard of being charged for a receipt...is this common?
In the past, I've just written a new receipt with "duplicate" in big, clear letters and made sure my accountant knew as well so it didn't get added twice.
I don't say this to be mean but.....In regard to the past, it would benefit you to let it go. Unfortunately, they were allowed to treat you like that because you allowed it. In the future do not do things for families unless you want too to be kind. Don't allow someone to walk over you and than hold it against them....unless you told them this would not be tolerated and they continued to do so. In that case, you should have terminated immediately than.