Except by default, extensions are not enabled in Incognito mode unless you specifically tell your browser to allow it.

On top of that, if a browsers incognito has the same browser ID of the non-incognito version, that's probably not good. I would expect a browser to randomize any unique information like that when launching a private window.

So all you've got, as a savvy tracker, is the same aspect ratio, which, big deal, not like there's a huge selection of monitor sizes, and the same IP address, which, again, big deal, since any one client IP can have an almost unlimited number of users behind it.

You can presume it's the same person, but bluntly, that's a wild guess. It could be a visitor, or a different user logged into the same computer or another computer at the same location with the same (or at least a similar in resolution) screen. It's honestly a crapshoot. Assuming that's the person you know accesses your site from that IP is a bit of a stretch.

Any tracking cookies created in an Incognito or private window are going to get shredded when the window is closed, as long as the browser is doing what it's supposed to do.