When a spell or ability asks for multiple targets while only using the word “target” once, you must choose different targets.  Some spells use the word “target” more than once, but specify “another target,” and the same concept applies.

In the case of Inferno Titan‘s triggered ability, the number of targets chosen is locked in when its controller puts that ability on the stack.  If two or three targets are chosen, they must be different targets.  Spellskite can never force itself to be more than one of those targets, just like you couldn’t choose it as the same target twice when casting Arc Trail.  A few spells use target more than once and don’t require those targets to be different.  So for something like Agony Warp, you could change both targets to Spellskite, though it still requires two activations of Spellskite’s ability if it wasn’t already a target.
 
Today’s Rules Tip was written by Josh Stansfield, a level 1 judge from Orange, California, United States.