I also think that it's more a language issue than a problem with the description. But having said that, I wonder: is the note about counting multiples of 15 only once even needed? I think it's a part of the challenge, and it should become clear after analyzing the problem statement and examples?

Sorry, but that seems to be a problem on your part, not in the instructions. I've changed the for a, but I still don't see how you could read this:

i thought that only 15 should be added to the sum excluding all commom multiple of 3 and 5.

 If a number is a multiple of both 3 and 5, only count it once.

         ^---------------------------------------------^       

15 is a multiple of both 3 and 5, but you add it only once, the same for 30, 45 and so on.

Note: If the number is a multiple of both 3 and 5, only count it once.
this note is misleading. i thought that only 15 should be added to the sum excluding all commom multiple of 3 and 5.