Which note to double

Root position: double the root

One note of the triad must appear twice. In root position the safest choice is the root: it is the most stable note, so doubling it makes the chord sound full and settled.

Every chord you write should have all three of its notes. The one exception: in root position you may leave out the 5th and use the root three times, most often in the final I of a perfect cadence, when the voices could not otherwise move well. Never leave out the 3rd: it decides whether the chord is major or minor.