Till now, i have played this card so: if it triggers his effect, you dont deal to him any damage, you 'change' this damage in a character you control.

Forced: When Thrashing Tentacle is attacked, discard the top card of the encounter deck. If that card has a shadow effect or is a Tentacle enemy, deal the damage from the attack to 1 character an attacking player controls (ignoring defense).

I have seen a guy playing so: you deal damage to your character, ok, but it is not 'changing', the first damage is dealt anyway tto Thrashing Tentacle.

It is ok?

The damage gets redirected-- it says "deal the damage from the attack," and there is only one set of "damage from the attack." So deal damage to your character, and not to the tentacle... don't double the amount of damage being dealt.

Ok, as i guess. Thanks for the answer.