I heard that the good rule of thumb is to be concerned about unique ones. It much less probable that you develop exactly same looking cancer in two unrelated spots.

Yes we call this the "Ugly Duckling" sign

Lots of models out there but I would not trust any for diagnosis without review of a dermatologist yet. The challenge is unanticipated edge cases and managing risk/liability/regulation. I have no doubt that if a major AI company focused on this problem then these issues could be overcome with current technology but perhaps the market is not big enough to justify the investment required.