As has been discussed in another thread, it seems silly to not allow the heroes to even move to a dungeon location on the Terrinoth map even though they have explored the dungeon.
I'm pretty sure they should still be able to move to the location, they just dont get the XP for finding it, and they cant go in and play the dungeon levels.
However, could they actually enter the dungeon just to use the glyphs of transport to go back to Tamalir? I'm guessing not, since this isnt mentioned anywhere in the rules, but if they cant, the heroes could find themselves in a situation where they are surrounded by dungeons they have already explored and a long way from home.
Is the only way to get back to Tamalir to enter a dungeon they havent explored, and then when they exit, choose to return to Tamalir?
If this is the case (which I think it is), the heroes need to be really careful if the OL has any Lt's near Tamalir.