some quest to ask ...

1. Alchemis cant use 2 same buff potion?

2. if cant , how about a potion +70int & 1 potion +50 all stat . will it become 120 int ? [ 2 potion no same]

The answer to 1 is correct. For example, you cannot use two of the +EXP potions for double the effect. I'm unsure about the second question though. I would imagine it's possible, since they're two different potions, even though one stat is affected twice.

Two different potions work under two conditions.

1) You're an alchemist.
2) The potion says alchemists can stack two at once.

And you have to be the same level as it is to make it.