I have a silly question i hope you can help me with it 
The question is: What happens when users of the same site use similar keys??
Thanks in advance
Depends on what you mean with users using the same key.
If you mean users using the same Forge app? The Forge app will access the same value for the same key.
If you mean users using different Forge apps, the Forge apps will access different values for the same key.