Hi,
Odoo uses the External ID (also known as XML ID) to 
identify and update existing records. If you leave this field blank or 
avoid it entirely, Odoo will not link the data to any existing record, 
and It will create a new record every time.
Even with no 
External ID, Odoo may still raise an error if the model has Unique 
constraints on fields like email, phone, and name (depending on the 
model). You’ll need to remove those constraints (inheriting and 
overriding the model) or ensure unique values.
Hope it helps.