You DO get exp from challenging the same quest more than once. In fact, even when exiting quest when you are not finished, you still keep your gained experience, gold, and/or items.
If you want to level quick, or power level, just keep doing the Evil Twins Battle map. Each mage, archer, or warrior enemy gives:
-30 experience crystals
-20 gold coins per drop
The mages map gives 300 exp crystals
The warriors map gives 210 exp crystals
The archers map gives 240 exp crystals
The mixed-enemies map gives 210 exp crystals
EDIT: Each quests overkills it's predecessor exp crystal gain-wise. It is recommended that you just do the latest quest easiest and fastest for you.