This setup is not common, but it is also less rare than you think. It's possible in some scenarios, I can think of the following:
- Users having more than one connection at home (for reliability or other reasons) and who use a random one for each connection (possible to set it up that way)
- Using TOR or other VPN/anonymation services can cause users to suddenly switch IP
- Some strange corporate/offices environments
It's something that you should definitely take into account. When I was working for a very big website I implemented a SSO (Single Sign-on) system that assumed the IP of two subsequent web requests was the same if the user was the same.
Surprisingly to me at the time, dozens of users complained of things being randomly broken, and after some investigation I discovered that all of them had more than one IP. Granted, we received millions of visitors so the percentage is very very tiny, but those people are out there and they can be "legit".