Warn if someone loaded a module at the same path twice

[justinbmeyer]

Today I had someone import models/todo and ./models/todo. This caused a situation where CanJS's real-time didn't seem to work. This was because one part of the page was using one Todo export and other was using a different Todo export. It took me a while to figure this was because there were two Todo types.

I'd like to propose warning someone if a file at a path has been loaded twice.

This can be done by keeping a side table storing paths to module names. If two modules are added to this table, we should warn.

This warning would have made the problem immediately obvious.

I think there are very few reasons the same module might want to be executed twice. I remember @matthewp talking about this. I don't think these cases are common enough where a warning wouldn't be helpful for a majority of users.

[matthewp]

👍 seems reasonable.