update_post_meta adding to array instead of replacing value

From the code that you shared, it follows that the one responsible for duplicating the custom field low_jobs can be the line below FeedWordPress::diagnostic().

If the key low_jobs is encountered in the loop and:

• $value == 'under 100' then field will be updated
• $value != 'under 100' AND $lowjobtrig == 0 then if the meta low_jobs doesn’t exist, it will be created, but $lowjobtrig is always 1
• in other cases meta low_jobs will be created, even if it already exists

So every time, the key is low_jobs and the value is different from under 100 this line

add_post_meta($postId, $key, 'OVER 100', true); 

will not be executed. Instead, this code will be executed:

FeedWordPress::diagnostic( ... );
add_post_meta($postId, $key, $value, /*unique=*/ false);  // <---