Which would be the most user-friendly way for something that may work or not work which may cause trouble to the user?

I'm building an Android app which deals with the list of contacts.

In my app the user selects a contact and has to do some things related to that contact for a period of time.

Problem is that the contact may change its phone number during the process, so the user would have to do it again from the beginning.

I've checked that's impossible to get which contact was a certain contact when initially selected, there are better or worse methods but on last instance none of them have a 100% of success.

Even them those methods might give on some very rare cases false positives, changing the contact to a completely different one, though that would be very rare, so I guess it's worth the risk.

If the user has to start the process again he might feel annoyed, but as I'm telling it is impossible to know if that new phone number is from the same contact or from another one with security.

I've come to the conclusion that the best I can do is if the change isn't recognized by the app, showing that the data might be incorrect, so he may have to reselect contact, but that would suppose to have to start again.

Allowing the user to continue is a possibility, but it would open a lot for the user to cheat, so I'm not counting on it.

Which would be the best way to handle this situation?