So I just tried this with 4.12.
I was able to have an Interface like so in C++…
UMyInterface2 : UMyInterface 1 {…}
//with an event in the parent and an event in the child.
…and that would compile, but if you go into the editor and create any blueprints (using BlueprintNativeEvent), and add the child interface, any time you try to compile that blueprint with the child event placed in the event graph, it will say that there is a node of the same name that exists already and won’t compile. This does NOT happen so long as you don’t try to actually create the child event and only reference the events that are native to the child. If you remove the child and add the parent interface, then it can compile with the parent event without issue.
I haven’t tried it yet with regular functions or in C++ yet. I suspect functions in BP would work the same way and not compile, but perhaps the C++ side won’t complain?