The problem is automation results in errors 80% of times: "Trying to add a non-unique value to a field marked unique".
I guess this happens because at the point the scope for automation's actions is defined, the field "DestinationExists?" is still "False".
I think you might be overthinking this? Sounds like in the table where you're adding records, you have a field that's set to accept only unique values, but your automation is attempting to add records that have non-unique (blank?) values in that field. Is that a direction you looked when troubleshooting your issue here?
