AJAX pagination offset returns same post on adjacent pages

I have a simple AJAX pagination that seemed to work just fine, calling one WordPress page (not ideal, I know) with a pagenum and a sort order arguments and it returned 4 custom posts per call. The problem I found while testing it is that if the items are ordered like this in the db:

item1: 4 votes
item2: 3 votes
item3: 3 votes
item4: 3 votes
item5: 3 votes
item6: 2 votes

…there’s a chance that any one of items with 3 votes in custom field will display as the first item on page2 of the pagination. This effectively erases one of the items from listing, and I can’t seem to find a way to query get_posts properly.

This is how it looks now:

elseif($savjet_order == 'votes') :
$offset = (($savjet_per_page * $savjet_page) /* - $savjet_per_page*/ );
//$offset = $offset < 0 ? 0 :  $offset;
$args = array(
'numberposts'     => $savjet_per_page ,     
'offset' => $offset ,
'meta_key' => 'wpcf-glasova',
'orderby'         => 'meta_value_num post_date',
'order'           => 'DESC',   
//'paged' => $savjet_page , 
'post_type'       => 'savjet',    
'post_status'     => 'publish'
/*'suppress_filters' => true*/ ); 

$savjeti = get_posts($args);

…adding post_date (or even date) to orderby clause doesn’t seem to change anything (‘date’ seems to make it worse by making the list ascend even though the order is DESC).

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