It's caused by a plugin, not by the file showpost.php.
If you look at line 258 in showpost.php you will see that the showpost_post hook is called. Go to your plugin manager and find a plugin that uses the showpost_post hook. If there's more than one plugin using it, disable them one by one until you find which one is causing it.
Once you found the culprit, past the code of the hook here and we can take a look.
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