[RollTi]

use a global variable

something like

pawn Код:

new MegaJump[MAX_PLAYERS];

pawn Код:

CMD:megajump(playerid, params[])
{
    if(MegaJump[playerid] == 0)
    {
         new Float:P[3];
         GetPlayerPos(playerid,P[0],P[1],P[2]);
         GetPlayerVelocity(playerid,P[0],P[1],P[2]);
         SetPlayerVelocity(playerid,P[0],P[1],P[2]+5.0);
         MegaJump[playerid] = 1;
    } else if(MegaJump[playerid] == 1) {
         MegaJump[playerid] = 0;
    }
    return 1;
}

    if(PRESSED(KEY_JUMP))
    {
         if(MegaJump[playerid] == 0) return 0;
         new Float:P[3];
         GetPlayerPos(playerid,P[0],P[1],P[2]);
         GetPlayerVelocity(playerid,P[0],P[1],P[2]);
         SetPlayerVelocity(playerid,P[0],P[1],P[2]+5.0);
    }

Not sure if it works