When two of my favorite players are featured on the same card I always want two copies so each PC can house one. That's not always possible though, particularly in today's world of a bajillion parallels, many of which are very short-printed.
A perfect illustration is this 2009 Topps Triple Threads Triple Relic with jersey swatches from Matt Kemp and Russell Martin sandwiching Manny Ramirez. It's numbered out of a scant 18 - 09/18 in my case - and is the emerald parallel. I'm not likely to come across another copy and certainly not for the price I paid (~10 clams). It's headed for my Bison PC, as he's my number one priority. I have no idea when Kemp and the Dodgers wore wool throwbacks, but whatever.