[OciXCrom]

1.9 version:

#include <amxmodx>
#include <cstrike>

new bool:g_bBought[CsTeams]

public plugin_init()
{
    register_plugin("1 AWP per Team", "1.0", "OciXCrom")
    register_event("HLTV", "OnRoundStart", "a", "1=0", "2=0")
}

public OnRoundStart()
{
    g_bBought[CS_TEAM_T] = false
    g_bBought[CS_TEAM_CT] = false
}

public CS_OnBuy(id, iItem)
{
    if(iItem == CSI_AWP)
    {
        new CsTeams:iTeam = cs_get_user_team(id)

        if(g_bBought[iTeam])
        {
            client_print(id, print_center, "Only 1 AWP per team is allowed! Try again next round.")
            return PLUGIN_HANDLED
        }

        g_bBought[iTeam] = true
    }

    return PLUGIN_CONTINUE
}

I'm really not willing to do the 1.8.2 one because the API doesn't provide an easy way of doing it.