Found it. (Destiny)

by bluerunner , Music City, Tuesday, August 05, 2014, 11:23 (3560 days ago) @ DaDerga

Thanks, electricpirate.
http://www.eurogamer.net/articles/2014-04-30-how-destiny-turns-into-diablo-when-you-hit-the-level-cap

Destiny allows you to freely swap your ability choices, but according to Green there comes a point when you can lock them in. This gives your Guardian benefits, such as a stat boost. At this point you get a copy of the same focus, but that copy levels up much faster, thus allowing experimentation.

So based on that and the quote about only 2 foci, the third slot could indeed be just for a copy. If that's the case, does that mean we can only copy one focus?