If a customer has paid an invoice twice, or paid a larger amount than needed you can easily solve this in Moment.
Because Moment is not an accounting software, the main concern will be to make sure that the customer ledger balances out. To solve this, click on the selected invoice and go to Payments and write-offs. You will then see a page similar to this:
In this example the amount that has been paid in excess of the total is 31 935 NOK. You can then click on "New payment" In that section, add a new negative payment (this will come automatically if Moment has already registered an invoice as paid twice) Write a quick note (to keep track of changes) and click "save".
This will make the customer ledger for this customer balance out. Off course, this process merely balances the account, but no reimbursement or credit has been made to the customer. The proper bookkeeping entry and payment has to be made through the accounting software.