Ace rerolls ?

[fula farbrorn]

When you use ACE to reroll dice when defending, can you reroll a die that you already rerolled ?

For example i roll 4 die, i need to soak 4 but i got 10 ace, so i start rerolling until i got all 4 soaked ?

Or can each die only be rerolled once ?, that is you roll your 4 dice and then you can reroll each of those one time each.