QB Issue Resolution:

I would void and put a memo note in it that it was a duplicate. But you might just make yourself a note first depending on which one the customer pays. If you delete or void  the wrong one and receive payment you will have to bring back or recopy. Make certain the customer knows which is which, maybe if you void, save a off and send it to the customer with apologies.

Voiding and deleting have same effect in current year. Voiding allows a record to be kept of what originally was there (In memo) and if you void something from a prior year QB will offer to create an adjusting entry in current year in case the item was included in p&l

Resolution for Issue 'I invoiced a customer twice for the same service. Do I delete or void the duplicate invoice? When would you delete or void -what is the difference?' available: Yes (Solved).
Source: Intuit Community forum.

I invoiced a customer twice for the same service. Do I delete or void the duplicate invoice? When would you delete or void -what is the difference?: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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