You want to work with one reputable mystery shopping program once you've found the right fit, that is a given. But this is not a rule that applies to each individual mystery shopper. It is easy to be tempted - if you get a useful report, you may feel better sticking to what you know works.
You'd be surprised at the frequency that a repeat mystery shopper is requested. It might be stating the obvious, but here are the reasons why that is a bad idea: