I think it is that if the key was repeated like if it was multiple keystrokes, it would count/function as getting more than one action per keystroke. Like an autofire function on some game console controllers, where you can hold down a button and it will treat it as multiple rapid presses of the button.
If you could hold down a single key and repeatedly trigger an action from the left side of WoW's key binding screen, that would break the rules.
But you can't do that this way. (Or if you can, you can also do it with an ordinary keyboard.)
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