[exotica_core] Property: Warn and override if double-adding the same property
[exotica_core] Property: Warn and override if double-adding the same property

This fixes a bug where one could specify the same property several times
but it would not correctly override or warn. This led to unexpected
behaviour.

openhumanoids/exotica,openhumanoids/exotica,openhumanoids/exotica,openhumanoids/exotica