QB Issue Resolution:

Currently, there isn’t an option in place to warn if duplicate customer invoice number is used with the popup message window in QBO at this time. They have warning notice for Vendor Bill (from the Advanced tab), but not for Customer Invoice.

Usually, if you turn-off “Custom transaction numbers” from the Sales settings, you should not see duplicate customer invoice numbers.

Resolution for Issue 'Duplicate Invoice numbers' available: Yes (Solved).
Source: Intuit Community forum.

Duplicate Invoice numbers: 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.

Support for duplicate invoice
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Error Recovery for duplicate invoice [Canada]
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