Yes there is a rule. From the SCDSL Rules 2017:
REFEREE GAME RESPONSIBILITIES:
- Check player’s equipment (FIFA Law 4): cleats, shin guards, and other uniform requirements.
- Any player without proper equipment will not be allowed to play.
- Check that the SCDSL Official Game Report is filled out completely and signed by a coach or manager from each team before the game.
- Check that each player on the lineup sheet has a valid Cal South player pass/card.
- Check that each player and coach is affiliated with the club.
- Check that player’s jersey numbers and names are listed on the lineup sheet and that no two players on a given team have the same number.
- Insure that all players being checked-in match the picture on the player card.
- Check that each team has a coach with a valid Cal South coaching pass. A minimum of an "E" License is mandatory for all competitive coaches. Any team that does not have person with a valid Cal South coaching pass shall forfeit the game. The “F” license is not a recognized license in the SCDSL.
The protocol is simple, one of the players must put on a new jersey with a number not already on the lineup sheet OR alter their jersey with tape (21 now becomes 121 or 211 or 81 or 27). Simple fix.
If there is no tape or no additional jersey then one of the players, by rule, must sit out the game. Now, if the game lineup was checked and all jersey's were different AND THEN a player enters the field with an entirely different jersey conflicting with the jersey of another player, then we stop the game, send the player off to fix her/his equipment, and the player can reenter with the referee's permission at the next stoppage of time.
At the end of the day ... we want these kids to play and will find every reasonable means to enable that to happen. This is youth soccer after all. The wins and losses at league mean very little in the grand scheme.