A command block will run a command once, but never again, even after being repowered, or after changing the command inside the command block.

Here is the best proof I have. In Fig. 1, the button runs into wire displayed in Fig. 2 and Fig. 3, seen running into a series of command blocks. I have taken screenshots of the last command that was run on the first (which isn't broken for some reason) and last (which seems to be the only command block broken), and the times are totally off. (I am aware that this could have easily been faked, but it is the best proof I can provide).