Pings/Trackbacks: Avoid adding multiple `_pingme` and `_encloseme` meta...

Pings/Trackbacks: Avoid adding multiple `_pingme` and `_encloseme` meta entries to a post when it gets updated prior to pings being done.

Props rebasaurus, whyisjake

Fixes #48014

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git-svn-id: https://core.svn.wordpress.org/trunk@46224 1a063a9b-81f0-0310-95a4-ce76da25c4cd