Custom field. Control duplicates

Hi, I created a custom field of type text that I called the Tax Code - name field codice_fiscale_c in module Companies
I activated the “Merge Duplicates”
When I search for possible duplicates company I can not find the duplicate based on the field that I created because not listed.
Then if I create a company with the Tax Code field equal to that of another company already present does not create the message “company might be duplicated.”
How can I have control of the company can also duplicated through custom fields?
It seems that is the only control on the company name.
Thanks for the support

Hi,

I have the same problem, in the accounts module.
How do we avoid creating duplicate entries?

Regards,
Leonardo