At a recent AYSO tournament I noticed that on a number of teams, there would be two players with the same number. When I asked the referee officials about this, I was told that there was nothing in the Laws to preclude this. However, in other answers you have cited “ancient and well-established tradition” (my wording, actually) for common sense rules that are not explicitly spelled out. It seems to me that this would be such a rule, as the referee uses player numbers to record misconduct and, in AYSO, playing time. I would appreciate an official answer to cite for when, not if, this issue reappears.
USSF answer (July 14, 2008):
The Laws of the Game do not require player numbers; that is a function of the rules of the competition ()league, cup, tournament, or whatever else). It is indeed traditional and simple good sense that players wear individual numbers to distinguish themselves from one another; however, some competitions may not require either rosters or even the numbers to tie them to that roster. If the competition does require rosters, that suggests it also requires numbers. If that is the case, then the problem will die away if the referee enforces the requirement. If either or both of these is not required, then the referee’s only practical recourse is to ensure that he or she obtains a name from any carded player (and affixes in his/her consciousness some player features to assist in tracking the person).