Depending on the library, including it more than once could have undesired effects.
Think of it like this, if you have a script that binds a click event to a button, and you include that script twice, those actions will be ran twice when the button is clicked.
You could write a simple function that you call to load a script and have it keep track of files that have already been loaded. Or, I’m sure you could probably use a pre-existing JS loader such as LabJS and modify it.