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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.
If a triggered ability is linked to a second ability, additional instances of that triggered ability are also linked to that second ability. If the second ability refers to "the exiled card," it refers to all cards exiled by instances of the triggered ability.
If an Ally entering at the same time as Katara (including Katara herself) causes a triggered ability of an Ally you control to trigger, that ability triggers an additional time.
Replacement effects are unaffected by Katara's ability. For example, an Ally that enters with one +1/+1 counter on it won't receive an additional +1/+1 counter.
In some cases involving linked abilities, an ability requires information about "the exiled card." When this happens, the ability gets multiple answers. If these answers are being used to determine the value of a variable, the sum is used.
