Can you do the same shop more than once?

Yes, depending on the job it will tell you the frequency in which you can complete them like every 90 days ect.

I did one and I couldn’t go to the same store for 90 days.

Yes, I have done several restaurant shops for the same chain but at different locations. I couldn't repeat the same location, unfortunately, but that makes sense considering the staff might recognize me! However, I did a shop for a national gym chain and couldn't do the same shop at a different location, and it's not clear to me why.

If you had to provide any personal information during the shop, they may be able to track you in their system at any location.