Sometimes I see in the instructions, specifying to NOT do it again if you've done it before, or it would be rejected. Typically for things like surveys. Other times, it says nothing in the instructions about it, and I don't recognize it as something I've done before until after I begin the survey, and realize I did the exact same survey a couple days before. In that case, I'll return it in case they notice I've done it before and reject the hit. I imagine the same person doing the survey over and over is useless for their data anyway. Seems like it should be possible to prevent the same person from being able to accept the same hit twice though, shouldn't it? There's another one that I've done but still see popping up every few days. The one where they want a picture taken of a closeup of an eye. I felt safe repeating that one because it does say to take a new picture, not use a picture from a previous hit. So the pictures won't be identical anway. I don't want to risk getting rejected for having a duplicate hit though. Just makes me nervous how often I see stuff appear even though I've done it. At a certain point, I won't necessarily remember.