use "xxx[]"
as name of the field in formdata (you will get an array of - stringified objects - in you case)
so within your loop
$('.tag-form').each(function(i){
article = $(this).find('input[name="article"]').val();
gender = $(this).find('input[name="gender"]').val();
brand = $(this).find('input[name="brand"]').val();
this_tag = new Array();
this_tag.article = article;
this_tag.gender = gender;
this_tag.brand = brand;
//tags.push(this_tag);
formdata.append('tags[]', this_tag);
...