Undulating Tides is an azerite power on some of my items. It gives this:
"Your damaging spells and abilities have a chance to crash a wave upon your target for 5701 Frost damage. When your health drops below 50%, gain a shield absorbing 44704 damage within 8 sec. When this occurs, Undulating Tides cannot trigger again for 45 sec."
The question is, should I use it on multiple items and I get double or triple bonus for both shield and damage?
Yes, Azerite Traits often stack as does Undulating Tides, usually it is better to stack your azerite trait 3x but some circumstances you might want 2x / 1x - particularly when a trait gives you a unique ability, for example a destro lock can proc a instant cast incinerate due to a trait(just an example!).
It’s all situational, I have spent a small fortune in BFA just changing traits according to content I’m doing but often stacking the same trait 3x in each ring is the stronger boost.