Even though identical twins are sometimes distinguishable by the human eye, facial recognition algorithms will not be able to pinpoint and distinguish the physical traits between each of them.
Although facial recognition algorithms look for similarities when returning candidates in a search, significant facial changes such as weight gain or loss do affect matching accuracy by reducing the matching accuracy overall.
Images that are not taken in a controlled environment, like CCTV, ATM or social media pictures, are still usable thanks to advances in facial recognition technology.
No. All facial recognition matches are considered possible and arrests should not be made based solely off of facial recognition software results.
In most cases, mugshots are a great image source for facial recognition.