There is no regulation that prevents the same soldier from being picked for any detail. If you feel that the soldier is being picked because of Race, Color, Gender, etc. either positively or negatively then it is an EO/SHARP violation and that can be found in AR 600-20. If you feel that the soldier is being chosen as reprisal for an incident then it is an issue to bring up with your IG. If you feel they are being chosen as a part of an "initiation" then it is Hazing and also covered by AR 600-20.