Unique identification

I have oberved that more  than one non optional
'assigning_identification' templates can be assigned to a specific
entity/template.
How are the one(s) that are used for unique identification distinguished
from the additional  'assigning_identification' templates.
 
This information must be unambigous.
Otherwise a DEX population cannot be loaded/merged into an integrated
repository.
Duplicates/redundancy will be the consequence.
Applications will also need to know which ones are the unique
identification(s) whenever looking up
items (for instance PARTs) out of the blue.