If I get a badge once, can I get another if I do the same thing again?
Say for example the "Stellar Question" badge:
If I have 2 questions favorited by 100 or more users, do I get the badge twice?
Depends on the badge. Badges such as Autobiographer(awarded when the user completes all user profile fields) are not awarded everytime the user completes all fields(if he removes/edits any field).
However, badges such as Stellar question (Question favorited by 100 users) awarded to the OP everytime a (new) question is favourited by 100 users.