asi deberia ser
This is a bad misuse of terminology.

"Stacking" refers to the ability to multiply a buff over and over, up to a certain limit. For example, warriors sunder armor stacks up to 5 times, and mages fire vulnerability from scorch stacks up to 5 times.

This buff does NOT stack. If it procs again while the 15s buff is up, then the buff is RENEWED to 15s, but the effect does not multiply.

What I think people are talking about is that it can be active once per weapon, so if you have two weapons with it on, then it can be active as two separate buffs with separate timers. This is not what most people think of when you say "it stacks".