The document number can be repeated if the Allow Override preference is checked.
It is possible to have a duplicate of transaction numbers when the company allows overriding the auto-generated transaction number on Setup > Company > Auto-Generated Numbers > Document Numbers >Allow Override.
a. When users create the same transaction and the Duplicate Number Warnings preference from Home > Set Preferences > Transactions > Warnings > Duplicate Number Warnings checkbox is set to No Warnings.
b.When users create the same transaction, and the Duplicate Number Warnings preference from Home > Set Preferences > Transactions > Warnings > Duplicate Number Warnings checkbox is set to Warn but the user had previously marked and saved the Do not Show This Warning Again checkbox on Duplicate Number warning message page.