Some restaurants, notably in the US, pre-authorise your Revolut card with the total amount for your bill, for example $40. If you add a tip, a second charge including the tip may appear on your Revolut account - for example $47.50. This will appear in your transaction history as a double charge until the pre-authorisation is cancelled by the merchant or expired. This can take up to 15 days to be automatically credited back to your available balance.
If this is not the case, please attempt to resolve this issue with the merchant, and they will quickly issue a refund if having charged you twice by mistake.