Unable to edit/delete custom field

We have added a custom link field in Material request document linking to the related sales order.
As some user were forgetting to select the sales order or entering same so n° we have switched the field to unique.
however< since that time we are getting the following message

sales_order field cannot be set as unique in tabMaterial Request, as there are non-unique existing values

now we can’t uncheck the unique box or modify anything without getting this message
can you please help ?

I’d try to:

  1. locate these non-unique values
  2. manually eliminate duplicates (probably using the …-1 methodology that ERPNext applies for amended Sales Order, etc …)
  3. retry changing the custom fields properties to ‘unique’

Thank … all documents have already been processed and can’t therefore be amended or modified. i m only trying to remove the custom field property back to normal (unticking the unique box) but the same message come when unticking the unique check field.