It will happen to everyone at some point. You are going to find some people who seem like they might be your ancestors but then again, they might not be. How are you going to deal with this?
The one thing you are not going to do is assume anything and you are also not going to add them into your family tree and call them your ancestors. They need to go in a separate area where they will no doubt have quite a bit of company as time goes on.
Primary sources are the only true way to verify that people really are our ancestor but often when we get back several generations, there are not going to be any primary sources. Do two secondary sources make a primary source? I hate to say it but that depends. It depends how reliable the secondary sources are and how well they pass the proof test and how many of them there are.
Does the ancestor in question have a child of the name of your last known verified ancestor? That is a pretty good indicator. Are they living in the same area as the last known ancestor? Again that is pretty indicative. Are there any other families with the same last name anywhere nearby? If the answer is yes then the whole question becomes a lot more cloudy.
Depending on the time period and the location, you will need to go looking for other options. You need to eliminate some of the possible ancestors. There is no perfect answer to this problem and every case will be different. You are going to have to think outside the box and look for the hints that can send you in the correct direction.
One last note, never assume anything. We all know what that leads to!!