Overwriting configuration files is likely to happen if a
user syncs their home directories across computers. In this
case, the biggest risk is that all nodes will end up with
the same certificate and thus Device ID.
When the model prepares a folder for syncing, it checks to
see if the configuration files this instance is using are
getting synced. If the are getting synced, and they aren't
getting ignored, a warning is emitted. The model is used
so that when a new folder is added dynamically, a warning
is also emitted.
This will not prevent a user from shooting themselves in
the foot, and will not cover all cases (e.g. symlinks).
It should provide _something_ for many users in this
situation to go on, though.
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