Previously, if a script bound the same key more than once to different
functions (without removing the older bindings first), then pressing
the key triggered any of the bound functions arbitrarily[1].
Now the newest binding for the same key is always the active one.
If the newest binding is then removed - the second-newest will become
active, etc. (same mechanism as before, except that the active key
was not always the newest).
[1] The order was implementation-defined, however `for (name in obj)'
happens to iterate. With mujs that's currently alhabetic order,
and name is from mp.add_[forced_]key_binding(key, name...)
or generated internally if name is not provided. So a binding with
higher alphabetic `name' value had priority over lower name value.