We all think we might get outed as a shopper, but really there is no reason to worry.
At a popular fast casual restaurant shop often discussed here, I place my order, and a customer behind me places his. He's finished placing his about 60 seconds after me. I go to fill up my drink, while he stands at the counter and is in a position as if he is watching them like a hawk. I take a position a bit behind him, but in a good spot, where I too, can watch everything. Wait time starts approaching eight minutes. Gotta be my order coming.... no wait.... its his order? (His order also included what would be required items on a shop). Based on where we were on my stopwatch, and that his "clock" started about a minute after me, lets say, he is good under eight minutes. Order presenter cheerfully gives it to him. Of course, I still wait. After another two minutes, and now a total time of over 10 minutes. Now mine is ready. I get mine with virtually no response.
Other guy also sits and eats. In this time, a crew member walked over to him and asked him how everything was. I never got that. Pretty funny, his report would have been so much different then mine. Makes you wonder how much the good service is based on them thinking they know who you are. That guy left without going to the bathroom, though. So they weren't double shopped!