Duplicate Key error when entering Employee DBA Instructions

Running E1 HCM v9.2.3

I'm getting a JAS_MSG317: Duplicate Key not allowed error when entering a new PDBA code for an existing employee.

Things to note:

• The PDBA code (1227) is entirely unique for this employee and is a valid value in our system
• Key of the V06106B is AB #, DBA Code, Date - Pay Starts, Transaction No. - Payroll (I see no duplication here whatever)
• The line I'm entering is highlighted in red, however, if I click "Go to Error" it takes me to a different line. Even on that line there is no duplicate key.
• The user changed the Voucher Flag from Y to N and the record saved. (I don't know what effect this has other than the obvious one that a voucher won't be generated from payroll). Why would this "fix" the issue?