Just some quick advice: don't deal with doppelgangers.
If someone is living a double life, they can't be trusted. It will cause problems and you'll have frustrating experiences. If someone is not being honest about something, it's the same as lying. Even if that person is telling you the truth and being honest with you, if they are hiding it from someone else they are still lying--they're just asking you to be a part of that lie with them.
With regards to photography, it's best to insist on a model release form. You probably don't legally need one (depending on exactly what you plan to do with the photographs), but it makes it more clear to the person being photographed that you retain the rights to the images. If they say no to signing the form, it may be best to walk away from that assignment.
I've recently learned this from experience. Learn from my mistake, don't learn this the hard way.
A good and simple model release form can be found here. A more complex one can be found here. You can copy-and-paste into a text document and quickly make your own. It's not hard to do and it doesn't take much time.
Again, don't deal with doppelgangers and always insist on a signed model release form. This is for your benefit.