The reward for completing a level doesn’t change and you don’t get it again - you don’t gain additional items/gems for completing the same goal again unless the level a repeatable challenge with increasing difficulty. (No grinding)
The main reward in most levels is gems. Complete a number of levels and collect enough gems to buy items of value.
Check this for a list of items that you can get from levels:
http://discourse.codecombat.com/t/when-do-i-get-which-item-a-reference/2212?u=zuf