Template File Called Twice

I’m having an issue where single.php template file is being called twice, breaking custom functionality on single post pages. I’ve confirmed that the php file is indeed being loaded twice by adding an error_log statement to log the post id; it logs two ID’s: the correct post and the next post in chronological order. If viewing the most recent post, it only logs that one.

The furthest I could get in debugging was determining that it has something to do with .htaccess; when I reset permalinks to default it stopped loading the file twice.

I’ve searched and searched and could not gain any insight into why this is happening. Any help would be very much appreciated!