Fortifying an already fortified city (or placing a caravan in a city with a caravan)
Yes. This was covered by the FAQ, v2.0:

Q: Can a player disband an army figure in a city that already contains a fortification marker, or a scout in a city that already contains a caravan marker?
A: Yes. However, the player does not add an additional fortification or caravan marker, or gain an additional bonus.