I suppose you're making an racing mod or something? I've already have done a racing mod. I suggest you use an extra entity for players car, cause it's too messy with players. That's only a lame suggestion. I would finish my own mod, but there was a problem with entity stepsize, that's why i didn't publish it

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About your problem, try
PHP Code:
engclient_cmd(id,"-right")
or
PHP Code:
if((Buttons & IN_MOVERIGHT) || (Buttons & IN_MOVELEFT)) {
set_pev(id,pev_button,Buttons & ~IN_RIGHT & ~IN_LEFT)
}