I have managed to get a button to appear by adding this code to the FormViewDialog javascript function:
options.buttons.splice(2, 0, {
text: _t("Save & Duplicate"),
classes: "btn-primary",
click: function () {
},
});
However i'm not sure what it is needed in the click function in order to get it to duplicate the current element and open the pop up form.
Edit: This is for odoo 12
https://apps.odoo.com/apps/modules/12.0/jt_sale_order_line_views/