It's a common practice. I am not an MSC owner and have not been behind the scenes, so this is my speculation. I am sure the MSC has a little flexibility on breaking the shopper rotation requirements with their clients, but only turn to this, if needed.
The only concern you should have is if it's a shop where the associate would recognize you and create bias that would affect the report. So it'll depend on your situation. Sometimes being recognized doesn't necessarily jeopardize your project.. A good example where it can affect the outcome is a bar / restaurant that focuses on whether the shopper's ID'd or not. The shopper tries to 'blend' in the crowd; however, if you return to the location and the same server takes your order. Even though he doesn't recognize you as a shopper, he may recall checking your ID already and state that 'they remember you.'
Anyways, this kind of situation is a scheduler / shopper relationship building type of thing. You help them out and perform well, they'll make exceptions, give bonuses, etc. This is one of the types of perks that happens when you build your relationship with an MSC / scheduler. If it concerns you, just throw on a reminder that you're OK to accept, but will require a shopper rotation exception since you performed it 'recently.'
Hope that helps relieve your concerns a little!
Shopping the Greater Denver Area, Colorado Springs and in-between in Colorado. 33 year old male and willing to travel!