Problem 001 sol 6 project euler

[mr-propre-debug]

In the sol 6.py there are those lines :
while a < n:
if a % 3 == 0 or a % 5 == 0:
result += a
elif a % 15 == 0:
result -= a

The if line doesn't add a % 15 == 0 twice. I've tested with a list where multiples of 3 and 5 are added and 15 only appears once, so the solution isn't right.