I suspect that (unless you have a policy with teeth: see below) all this is really doing is incentivizing your students to lie.
What I mean by a policy with teeth is something like: you reserve the right to call in any student for an interview, during which you ask questions in order to determine whether they actually did (and understand) the work they claim to have done, and if you are not convinced, their mark on the work concerned is determined by what they demonstrated on the interview, which may well be a failing mark.
This of course takes time and effort, when done properly, and once again is only feasible in unusually small classes.