Replacement effects are unaffected by Felix’s last ability. For example, if Charging Tuskodon (“If Charging Tuskodon would deal combat damage to a player, it deals double that damage to that player instead”) deals combat damage to a player, Felix’s ability won’t cause the damage it deals to be doubled an additional time.

Triggered abilities use the word “when,” “whenever,” or “at.” They’re often written as “[Trigger condition], [effect].” Some keyword abilities are triggered abilities and will have “when,” “whenever,” or “at the beginning of” in their reminder text.

If you somehow control two Felixes, a creature you control dealing combat damage to a player causes abilities of permanents you control to trigger three times. A third Felix causes such abilities to trigger four times, a fourth causes such abilities to trigger five times, and so on.

Felix’s last ability affects only triggered abilities whose trigger conditions refer specifically to damage being dealt. It does not affect triggered abilities that would trigger because of the results of that damage. For example, if you control Felix as well as an Ajani’s Pridemate (“Whenever you gain life, put a +1/+1 counter on Ajani’s Pridemate”) and a creature you control with lifelink deals combat damage to a player, the triggered ability of Ajani’s Pridemate will still trigger only once.

Felix’s last ability doesn’t copy the triggered ability; it just causes the ability to trigger an additional time. 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.