Does it required a slugfield's must be unique in django?
Making slug unique ensures that it's unique in database so you can rely on it with no worries in your python/sql code. So it does make sense and that is best practice in general.
Concerning IntegrityError you need to provide more details. I'm using such fields with no problems.
im using slugfield in django model's and i've set slugfield to be unique for a reason so that my post should be unique, my slug include's a combination of title and it's object primary key 
my slugfield's are generating a slug's as an example like: 

slug url : whats-your-favourite-character-from-the-defenders-1

and the number at the end of my slug represent's primary key ' as you see primary key in my slug which already make's an url unique , so does it make sense that i should use unique attribute in slugfield !
because my problem is when i update my existed object it throws an error 
IntegrityError: UNIQUE constraint failed: polls_question.id