If the IP addresses are at different times then it is possible. Most users will have dynamic IP addresses so every time they reboot their router they will be given a new IP address. Once the original IP address is released then it is available to be allocated to another user.
So to answer your question
Piscator wrote:what are the chances of a user being assigned the same ip address by their isp as another member.
Yes it is possible - but the odds are, I would say, quite long that you would get three members with the same IP address on one forum.