When Chris Pontius made the move from D.C. United to the Philadelphia Union this past winter, the 2009 first round draft pick became the latest in a long line of players to have played for both clubs. Since the Union came into the league in 2010, there have been 14 players who have played, or more accurately, have been on the rosters of each team. And that's just strictly players (more on that later).
7 of those players went directly from one team to the other. One of those players even went from the first club to the second, and then back to the first.