Feedback Plus shops the same locations more than once in the same month and so they need different shoppers. They also try to prevent the shopper from being identified by visiting locations too close to one another as sometimes employees work at multiple locations.
Most companies have some type of rotation in place. This offers the client more than one viewpoint and helps keep shoppers from being identified. I have seen some rotation limits up to 1 full year.
President of the Independent Mystery Shoppers' Coalition.