(SOLVED) How do I make the potion level overlap and add up?

I have to make a mob that shoots arrows with effect. Each time these arrows hit the mob, the effect will be summed up, and not just resumed. That is, if an arrow with effect level 1 hits a mob and transmits the effect to the mob, then the next arrow with the same level 1 will also transmit the effect to the mob, then the mob will have 2 effect level. 

If it's more complicated, is it possible to add duration instead of summing effect levels? That is, if the same arrow hits the enemy and gives him an effect for 10 seconds and the second arrow hits the same mob, then the duration of its effect will increase by 10. Is it possible?

I don't really understand English and use a translator, so maybe everything you read may be confusing, I'm sorry.