Expected singleton means that there are multiple record that successfully fetch but python is unable to show them. You need to iterate it so it can be shown one by one simply using "for" iteration. 

The code below may not fully works but the idea is suggesting you to use "for iteration" to show fetched record from ir.attachment.

if line.purchase_order_id and line.purchase_order_id not in old_orders:    
    for record in self.env['ir.attachment'].search([('res_model', '=', 'purchase.order'),('res_id', '=',line.purchase_order_id.id),]):

    record.copy(default={
            'res_model': 'account.move',
            'res_id': line.move_id.id,
    })

    return line

best regards,

Altela (altelasoftware.com)