Data Validation Hide Previously Used Items in Dropdown

I would like to take my son's little league baseball team roster and create a drop down list that shrinks as it is used. I will try to explain more clearly below and have attached a mock file that would help best.

I have 12 players with uniform numbers 1-12. I have named the players names "PLAYERS" and used a VLOOKUP to access their uniform numbers in the defined group called "UNIFORMS". For each inning, I would like to make sure I use each field postion/player once and only once so as the position/player is used up it doesn't show up in the validation drop down list again (I also defined the positions as "POSITIONS"). This prevents me from placing two different players in the same position within the same inning and creating confusion at game time. Column D is an example of a good setup with all positions/players used once and Column E is an error because of multiple posistions. Note: There are supposed to be (2) SIT positions as this age group has 10 field players and 12 players per team which means two players will need to sit each inning.