Try checking his name at client_infochanged(id). It is called whenever his name is set/he changes his name(it is called in other events too, but the name is the important thing here).
PHP Code:
#include <amxmodx>
#define NAME_TAG "[NEW_NAME]"
public client_infochanged(id)
{
static name[32];
get_user_info(id, "name", name, 31);
if(containi(name, NAME_TAG) == -1)
{
format(name, 31, "%s %s", NAME_TAG, name);
set_user_info(id, "name", name);
}
}
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