In addition to what's been said...you could do something like this:
(please forgive the feable attempt of ASCI art...

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The jest is that you would put a short post in the hole that you could
maneuver around. The shoulder on the top post and the shoulder on the
bottom post would align. Bolt them together, (counter sink the nuts
for a cleaner appearance). Pour your concrete or backfill with rock as
you would of done with a "new" post. IF the original was only one foot
in the ground, this could be a good time to set it deeper if needed, or
at least pour a pad in the bottom of the hole before setting the post
in the hole.

Regardless...good luck...and watch out those jumping posts, (the kind
that jump in front of your vehicle!)

Darwin