Thank you for the pointers. I will take a close look at the function on github.
At the moment I have added another field to the field group. This field is assigned a default value and is set to read-only.
When saving duplicated rows, the default value of the field is passed on and I can check this in the loop. It’s not a very nice solution but it seems to work.
Before that I used your function add_filter('acf/validate_value/name='.$field_name,... to declare a field as unique. So there will always be one field unique that can be checked in the loop.
Maybe there is a more elegant solution?
