If there is a Bloodletter in play, then my opponent deal 1 damage to Warrior Priests, then trigger Warrior Priests' ability(Forced: The first point of damage assigned to this unit each turn is redirected to one target unit in any battlefield) to redirect 1 damage to unit B.

A) How many damage Warrior Priests will recieve?

B)How many damage unit B will recieve?

About redirection (I don't have the rulebook here at work), I don't know if you need to have the damage assigned to your unit to trigger its effect (I think so). If it is the case, then thanks to the last FAQ, the moment the damage is assigned to your priest, you should resolve constant effects (like the bloodletter's) first, and then redirect one of the 2 damage to another unit (and that unit would have also his assigned damage doubled... not quite sure about that though.)

You do need to assign damage in order to redirect it, so the damage gets doubled as you said, before the Warrior Priests ability redirects one of those damage. The redirected damage is not doubled though, as it is not assigned (redirection happens right after damage is assigned), so it isn't doubled again by the Bloodletter.

Entropy42 said:

intresting, I just thought the redirect damage need to do assign-apply phase(but can't insert an action because it's non-combat damage), so the damage rediect to unit B will be double.

so, follow this explanation, the redirect damage won't be cancel by toughness( or even other cancal when assign effect) because it hasn't assgin phase.

Then how's the move damage effect work? need assign or not?

If I remember the FAQ correctly, effects like toughness ets. do not kick in during the assign phase, but as part of the apply phase (so after assign or redirection). Since the redirected damage is still applied, it will be subject to damage cancellation like toughness. I guess the redirect counts as the assignment.

I found the answer of move damage in FAQ

Moving Damage
Moving Damage is not considered dealing or assigning damage, and damage moved from one unit to another bypasses damage cancellation effects (like Toughness).

but still have question with redirect damage.

FAQ v1.5, p.7 reads:

Redirecting Damage (v1.2)
Some card effects allow for damage
to be redirected from one target to
another. Damage is always redirected
after it is assigned but right before it
is applied (this occurs during the same
timing as Toughness).

It's not written here, but since it happens after assignment and before application, I think the assignment still starts with the old unit but resolves (if you will) on the unit the damage is redirected to. So because of the redirection it is effectively assigned and then applied to another unit. Then, it is aplied. So all effects kicking in then (like toughness) resolve normally as part of 'apply'.