WordPress – admin_init hook being called multiple times?

I’ve got a problem on my WordPress site which I tracked down to admin_init being called twice.

I tried deactivating all the plugins and the problem goes away. I then reinstated each plugin and found which one is causing the problem.

My question is, how can I find the cause of the extra admin_init call ?

The plugin is bespoke to the site so I can’t simply replace with a different plugin and it’s one (blushes profusely) that I have muddled together myself.

Here’s what I’ve got so far after doing some research on the subject…

I’ve added
remove_filter("wp_head", "adjacent_posts_rel_link_wp_head",10);
to the functions.php file (although this may only stop the “init” hook being called twice and not the “admin_init”)?

I set up a function that sends me an email every time admin_init is called. Whilst generating the email I added the following code to it:

$backtrace = print_r(debug_backtrace(), true);

and then added $backtrace to the email text.

The first email I get has the full backtrace showing all the functions that have led up to the admin_init call originating from “admin-ajax.php”. The second admin_init call email doesn’t have any backtrace info at all ?

I also read that multiple calls to the init (and presumably admin_init hook) can be caused by images on the site (or other inclusions) returning a 404 error.

I have right-clicked and “inspected element” on the page to open the Firebug window and had a look through the tabs but I’m not 100% sure how to check for this. I didn’t see anything odd or 404’ing under any of the tabs but if someone could just clarify the procedure for checking this I’ll be able re-check for definite.

So… any other ideas as to why I’m getting this extra admin_init call and what I can check in the software to see where it’s being generated?

Thanks.