Then you'll either have to locally change the name of one of the files, or put one of them in a namespace and make sure all the references to it still points right.
It's always dangerous when you have generic sounding names in the global namespaces because of issues like this, but NGUI stays there to stay compatible with old users. If everything moved to UI.NGUI namespace, then 100 % of projects would break until they change every reference to that, and it has been made clear that this is not wanted (much as I would like it.

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