Multiple do_pings events shouldn't be scheduled

[ethitter]

Whenever _publish_post_hook() is called, a new do_pings event is scheduled in wp-cron. Since do_all_pings() (the callback hooked to do_pings by default) takes no arguments, and pings everything that needs pinging each time it runs, there's no need to schedule multiple of these events.

The potential for duplicate events has existed since its introduction in r3663. As part of #6966, r9181 aimed to reduce duplicate events. If a site's cron is backlogged, however, that workaround will fail as it only looks ahead.

[ethitter]

Only schedule an event if one doesn't exist

[SergeyBiryukov]

In 40054:

Pings/Trackbacks: Avoid scheduling multiple do_pings events in _publish_post_hook().

Props ethitter.
Fixes #39828.