Django objects.create() causing duplicate primary keys

I'm ruing load tests with JMeter on a Django project with a Postgres database. This has managed to cause duplicate primary key errors when creating M2M objects;

IntegrityError: duplicate key value violates unique constraint "entry_entrycharityenquiry_pkey"
DETAIL:  Key (id)=(108) already exists.

This is happening when trying to create the relationship to objects after a user has selected from a list;

for ec in to_add:
    EntryCharityEnquiry.objects.create(**kwargs)

I can't see a problem with creating objects in a loop, so where/what could the error come from?