wp_schedule_single_event running twice

I’ve boiled down the problem to as little code as possible. I’m doing this via a plugin:

class myclass {
  public function __construct() {
    ob_start();
    // Debugging cron sending two emails
    add_shortcode( 'rookm_test_cron', array($this, 'rookm_test_cron_shortcode'));
    add_action('rookm_run_cronjob',array( $this, 'rookm_action_run_cronjob' ));
  }
  public function rookm_test_cron_shortcode() {
    $this->_log('------- in rookm_test_cron_shortcode ----------');
    $now = time();
    $this->_log('$now is ' . $now);
    wp_schedule_single_event($now+10,'rookm_run_cronjob');
    return 'ran';
  }
  public function rookm_action_run_cronjob() {
    $this->_log('-----------in rookm_action_run_cronjob');
    $this->_log('time() is ' . time());
    wp_mail('[email protected]','test cron','body');
  }
}

Create a page and insert the shortcode to trigger everything.

When I click the page, the log shows:

04-May-2014 03:43:49 UTC] --- in rookm_test_cron_shortcode ----
[04-May-2014 03:43:49 UTC] $now is 1399175029

Then I load another page (this is a local dev env on my windows 7 machine, so no one else is accessing it) to trigger wp_cron….and the log shows:

[04-May-2014 03:44:02 UTC] -----------in rookm_action_run_cronjob
[04-May-2014 03:44:02 UTC] time() is 1399175042
[04-May-2014 03:44:04 UTC] -----------in rookm_action_run_cronjob
[04-May-2014 03:44:04 UTC] time() is 1399175044

I spit out get_option(‘cron’);
which looks like this:

[1399175039]=>
  array(1) {
    ["rookm_run_cronjob"]=>
    array(1) {
      ["40cd750bba9870f18aada2478b24840a"]=>
      array(2) {
        ["schedule"]=>
        bool(false)
        ["args"]=>
        array(0) {
        }
      }
    }
  }

Refresh the cronlist a short bit later and its gone.

So clearly the cron is getting added. Its getting scheduled, its getting executed, twice! And getting removed.

I repeated the same thing with wp_schedule_event with the same results.

I tried a bigger offset (time()+30 and time()+300) with the same results.

I added a debug_backtrace and I can see both calls are the same, coming from wp_cron.

This is a fresh install of WP3.9 with nothing else but my plugin and 2014 theme.

I checked google and only found this:

https://wordpress.stackexchange.com/questions/124894/wp-schedule-single-event-running-twice

which has no answers.

Help!