Does anyone who works in customer service know how to override a "duplicate transaction" error?

How many times has this happened to you? A customer asks to pay for two rooms or campsites, but your system doesn't let you ring them up symoytainaiously.

You process the transaction for the first room, then try to ring up the other room, only to find out that the sale cannot be processed because it's a duplicate transaction.
Does anyone who works in customer service know how to override a "duplicate transaction" error?
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