A IP adress is unique which is why it can't be shared by two different people at two diffent locations. It can be routed down to exact location and can only be shared by more than one computer at a time, if they are using the same router at the location. In a local network.
It would be really odd if two people at the same online forum got the exact same IP adress dedicated at a small period of time.
So actually sharing same IP isn't actually likely.
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