Hi @Bochie ,
I'm sorry my initial test wasn't based on 5000 items.
It seems that there is no way to set the uniqueness of a column when there are more than 5000 records.
But if we set the uniqueness of the column before there are more than 5000 records, then it will also work after more than 5000 records.
I'm not an expert on Sharepoint, so I suggest you go to the SharePoint forums for professional help.
Best Regards,
Wearsky