when you love someone and he loves you too but u're not in a relationship then one day he or she said to you that she or he loves someone else and loves you too in the same time WHAT WOULD YOU DO ????

I'd question whether it's really "love" ... Or just "attraction".

Love actually requires time and effort and maintaining a relationship.

I would tell Romeo to get lost, then I would RUN not walk to the nearest exit.

I'd make the decision easier for him and tell him to take a hike.

This is a normal situation if you are about 14 years old. 

It's hard at a young age for us humans to distinguish between love as a neurotransmitter storm that we react to vs an act that we actively choose to perform with another person as the beneficiary.

He does NOT love you OR her! He is thinking with his tiny head! You deserve someone who will treat you like their number one AND only one! Get rid of this selfish loser! YOU ARE a gem who deserves to be treasured!

Accept it and be with them anyway... A person to call your own is so hard to find these days...

Torn between two lovers? Screwed for actual life?

In this case, you have to give them enough space to decide. Obviously you don't want to be in a love triangle and they need to know that. Being clingy, jealous, etc., will only push them away. If the other person makes those mistakes, your odds are good of this person wanting to be with you instead.

i think that not love because she or he loved me from the beginning then she or he can't love anyone else

And what will you do here? In any case, she will choose. Well, or you can deprive her of the choice, get away from her.

that someone doesn't love either of you

sometime people say "i love you" when they dont understand what is it or when it's not even love. I believe love in the first sight, but i just dont believe in most of the situations when i think the expressions are way to easy or casual or not from a sincere place. Sometimes a person only say this because he or she has a certain purpose not because love.

you just need to be careful and dont fall for a simple trick way too easy. 

let's just say, if he or she loves you, you would know

it's hard to admit it's love when you are already questioning about what would you do, because if he or she really loves you, you won't have such worries. They would bring the answer to you.

Love happens once in life.

Well, first of all, you are not in a relationship and that
itself is very telling. If you both loved each other truly, you would have been
together, or at least there would not have been another one. Stop hurting
yourself. He should sort out his feelings and man up. Try focusing on a career
or something else. Divert your mind. You are still very young.