Code
T2101 not-unique
A write breaks a unique rule: a row with the same values in those columns is already there. A caller gets it with the status 409.
Details
fields names each column of the rule that was broken. Two rows that hold nothing in such a column do not break it.
The fix
Read the row that is there first, and change it in place of adding a second one. Where two rows may hold the same value, take the column out of unique.