Sorry if this has been asked before, but I'm confused about how many total rerolls one die can have.

Is this a once a die is rerolled it is locked (IA, and IIRC Rebellion) or not?

So, scenario:

Single Stormtrooper with an aim token. For the sake of the scenario lets say he's shooting at an T-47 or something else with armor and so needs a crit to hit

Rolls a Blank, spends his aim token.

a) This die can be rerolled a single time and the two remaining rerolls (from aim and precise) are lost regardless of the outcome of the initial reroll

b) I can spend all three rerolls one one die if I so choose, so I could, if I wanted to chance it roll blank(initial) -> hit -> blank -> crit

I've seen it played both ways and I'm trying to figure out what the correct way is.

Here's the rule you need, RRG page 13:

"A unit can reroll the same die multiple times by spending multiple aim tokens; however, each die can only be rerolled once per aim token."

There is no dice lock as in FFG's other games. Instead, there are just limited ways you get to reroll dice.

There is no dice lock as in FFG's other games. Instead, there are just limited ways you get to reroll dice.

Aim tokens allow you to reroll 2 dice. Precise increases the number of dice that can be rerolled.

This number is the dice, not how many times you can re-roll.
So roll in your attack, and spend an aim token. Choose how many dice you would like to re-roll, up to your limit (Aim(2) + Precise(x)). Pick up those dice and re-roll them.

After that, you may spend another aim token to repeat that step. There is no restriction on how many times a die can be re-rolled.