Hi,

You can trust the postgresql constrains, it wont raise without meeting the given condition. So here some how the constrains is triggered, may be the duplicate values get generated on the current commit/fly and on writing it to the db it is getting failed  and as it roll backed to its original state you may not be able to find the duplicated value.

In order to trace it, you can try with api constrains and see by adding some print/logger inside it to trace it. 

Thanks