Why does Lindy and Leti have the same mark on their foreheads?
Lindy's character was based on a fairy of same name from Lyrical Toy Box  (which is where the whole Nanoha franchise started). Here she has both dots and wings (which she displays in the Nanoha franchise when she is using her powers).

There doesn't seem to be a canon explanation why she has them in Nanoha franchise, but I wouldn't be surprised if she wasn't fully human, but partially different race.

And I would hypothetize that Leti's character design was based on Lindy's just to drive the point home that they are friends.