Why does hook_menu add my menu items multiple times?

Question

I'm using a hook_menu to add some custom menu items to my user menu. One is a link to an external page, one is a duplicate of the "node/add/discussion" page.

I'd expect these menu links to be added to my user menu once and only once. Instead, these links get added to my user menu once for every time I clear cache.

function MYMODULE_menu() {
    $items['external'] = array(
        'title' => 'External',
        'link_path' => 'http://www.example.com',
        'type' => MENU_NORMAL_ITEM,
        'menu_name' => 'user-menu'
    );
    $items['have_a_chat'] = array(
        'title' => 'Have a chat',
        'link_path' => 'node/add/discussion',
        'type' => MENU_NORMAL_ITEM,
        'access_callback' => 'user_is_logged_in',
        'menu_name' => 'user-menu'
    );
    return $items;
}

Exported from the database (SELECT * FROM menu_links WHERE menu_name = 'user-menu';) the menu looks like:

"menu_name","mlid","plid","link_path","router_path","link_title","options","module","hidden","external","has_children","expanded","weight","depth","customized","p1","p2","p3","p4","p5","p6","p7","p8","p9","updated"
"user-menu","2","0","user","user","User account","a:1:{s:5:""alter"";b:1;}","system","0","0","1","0","-10","1","0","2","0","0","0","0","0","0","0","0","0"
"user-menu","10","2","user/register","user/register","Create new account","a:0:{}","system","0","0","0","0","0","2","0","2","10","0","0","0","0","0","0","0","0"
"user-menu","14","0","user/login","user/login","Log in","a:0:{}","system","0","0","0","0","0","1","0","14","0","0","0","0","0","0","0","0","0"
"user-menu","15","0","user/logout","user/logout","Log out","a:0:{}","system","0","0","0","0","10","1","0","15","0","0","0","0","0","0","0","0","0"
"user-menu","20","2","user/password","user/password","Request new password","a:0:{}","system","-1","0","0","0","0","2","0","2","20","0","0","0","0","0","0","0","0"
"user-menu","336","0","profile-main","profile-main",,"a:0:{}","system","0","0","0","0","0","1","0","336","0","0","0","0","0","0","0","0","0"
"user-menu","1071","0","http://www.example.com",,"External","a:0:{}","system","0","1","0","0","0","1","0","1071","0","0","0","0","0","0","0","0","0"
"user-menu","1072","0","node/add/discussion","node/add/discussion","Have a chat","a:0:{}","system","0","0","0","0","0","1","0","1072","0","0","0","0","0","0","0","0","0"
"user-menu","1074","0","http://www.example.com",,"External","a:0:{}","system","0","1","0","0","0","1","0","1074","0","0","0","0","0","0","0","0","0"
"user-menu","1075","0","node/add/discussion","node/add/discussion","Have a chat","a:0:{}","system","0","0","0","0","0","1","0","1075","0","0","0","0","0","0","0","0","0"
"user-menu","1076","0","http://www.example.com",,"External","a:0:{}","system","0","1","0","0","0","1","0","1076","0","0","0","0","0","0","0","0","0"
"user-menu","1077","0","node/add/discussion","node/add/discussion","Have a chat","a:0:{}","system","0","0","0","0","0","1","0","1077","0","0","0","0","0","0","0","0","0"
"user-menu","1078","0","http://www.example.com",,"External","a:0:{}","system","0","1","0","0","0","1","0","1078","0","0","0","0","0","0","0","0","0"
"user-menu","1079","0","node/add/discussion","node/add/discussion","Have a chat","a:0:{}","system","0","0","0","0","0","1","0","1079","0","0","0","0","0","0","0","0","0"

My Drupal version is 7.23.

One odd thing I notice is that "module" is listed as "system" despite the fact that I'm adding these links in a custom module.

How do I fix my code so that these menu links only get added once?

Answer 1

This was a problem with link_path as hinted at by @xurshid29 and @ARUN.

link_path "works" in that it inserts a link with the given path into the menu, but it also has the odd side effect of inserting multiple links into the menu - one for each cache clear - despite each link having the same key in $items. It's also undocumented, so I have no idea where I picked it up from.

Removing the link_path argument and replacing it with "page callback" fixes the problem of multiple links being inserted.

"Have a chat" was fixed to explicitly callback the "add discussion form":

$items['have_a_chat'] = array(
    'title' => 'Have a chat',
    'page callback' => 'node_add',
    'page arguments' => array('discussion'),
    'type' => MENU_NORMAL_ITEM,
    'access arguments' => array('create discussion content'),
    'menu_name' => 'user-menu'
);

And "external" was fixed to page callback a function that then calls drupal_goto.