[KnooL]

On top of your script:
new spawned[MAX_PLAYERS]; // this will make a slot for each player connected to the server.

OnPlayerSpawn:
spawned[playerid] = true;

Function:
if(spawned[playerid] == 0) return SendClientMessage(playerid,color,"You are not spawned");

OnPlayerDisconnect/Connect:
spawned[playerid] = false;

if you do: new spawned; it'll be just one slot and it won't be for everyone that's why it is mixing up