[Bf-funboard] Duplicate keyboard shortcut removing

[Gatis Kurzemnieks]

Hello everybody. It would be really nice feature for Blender preferences
Input editor to check for duplicate keyboard shortcuts when creating new or
changing existing. It could show a warning dialog informing user that the
keyboard shortcut is already assigned to some other function and offer
choices of "Cancel" and "Remove Old Shortcut". There should never be one
shortcut assigned to multiple commands - now it is possible but only the
last one works (As I understand).

Also there seems to be a bug when importing Key configuration on top of
existing,some shortcuts get duplicated and configuration becomes a mess.

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Gatis Kurzemnieks