[DahVid]

#include <amxmodx>
#include <amxmisc>
#include <engine>

public plugin_init()
{
    register_plugin("Hit notifications","0.1","DahVid")
    register_event("Damage", "got_hit", "b", "2!0")
    register_cvar("hn_redflash","1")
    register_cvar("hn_shake","1")
}

public got_hit(id)
{
    if(is_user_alive(id) && is_user_connected(id)) {
        if(get_cvar_num("hn_redflash")) {
            message_begin(MSG_ONE,get_user_msgid("ScreenFade"))
            write_short(1)
            write_short(1)
            write_short(1)
            write_byte(255)
            write_byte(0)
            write_byte(0)
            write_byte(80)
            message_end()
        }
        if(get_cvar_num("hn_shake")) {
            message_begin(MSG_ONE,get_user_msgid("ScreenShake"))
            write_short(10)
            write_short(10)
            write_short(10)
            message_end()
        }
    }
}

[XxAvalanchexX]

You send it to MSG_ONE but don't specify the one player to send it to.

message_begin(MSG_ONE,message,{0,0,0},player);