Katara, the Fearless

If you somehow control two copies of Katara, the Fearless, an Ally's triggered ability will trigger three times, not four.

Triggered abilities use the word "when," "whenever," or "at." They're often written as "[Trigger condition], [effect]." Some keyword abilities, such as prowess, are triggered abilities and will have "when," "whenever," or "at" in their reminder text.

Katara's effect doesn't copy the triggered ability; it just causes the ability to trigger twice. Any choices made as you put the ability onto the stack, such as modes and targets, are made separately for each instance of the ability. Any choices made on resolution, such as whether to put counters on a permanent, are also made individually.