It depends on which camera app you're using, and on which version of iOS.
If you're using Camera.app (the inbuilt camera app) on iOS 10.2 or later:
Camera.app uses only the middle two lens identifications in the list above — the ones that say "iPhone 7 Plus back dual camera". Depending on which lens was used, you'll get either "3.99mm f/1.8" or "6.6mm f/2.8".
These usually correspond to 1x and 2x, but Camera.app may choose to use the wideangle lens for a 2x, and do a digital zoom. This happens when you take a 2x macro shot — the telephoto lens can't focus closely, so Camera.app takes a photo with the wideangle lens and does a digital zoom. Another situation is night photography, during which the tele lens doesn't work well .
So, 1x and 2x are really zoom factors, not lens choices. The lens choice is made by the OS. But if you want to find out what that was, you can look in the EXIF, which accurately captures which choice was made.
As you go through your photos, you'll find all three combinations of focal length and lens:
- Focal length: 28mm (in 35mm terms) / Lens: f/1.8
- Focal length: 63mm / Lens: f/2.8
- Focal length: 63mm / Lens: f/1.8
If you're using a third-party camera app on iOS 10.2:
Third-party apps, like ProCamera, let you force the wide lens to be used, or the tele lens, which result in the first two entries in the list (without the word "dual").
ProCamera also lets you leave the choice to the OS, in which case the EXIF has the same values as Camera.app.
If you're using an older version of iOS than 10.2, like 10.0 or 10.1:
These generate the last two entries in the list above, with "iSight Duo".
 Because of a small sensor — 1/3.6" vs 1.3", narrower aperture — f/2.8 vs f/2, lack of optical image stabilisation, and lower max ISO — 1216 vs 1760.