Step-1 install the account_invoice_copy module.
Step-2 After that customize .py file in account_invoice_copy module.
In below example, I have custmozied .py file. You can directly copy the below python code in your .py file -

 
    def copy(self, cr, uid, id, default=None, context=None):
        default = default or {}
        default.update({
            'state':'draft',
            'number':False,
            'move_id':False,
            'move_name':False,
            'internal_number': False,
            'period_id': False,
        })
        if 'date_invoice' not in default:
            default.update({
                'date_invoice':False
            })
        if 'date_due' not in default:
            default.update({
                'date_due':False
            }) 


        foo = self.browse(cr,uid,id)    
#        raise osv.except_osv('Object Error', 'Doc %s ' % str(foo.file_ids))

#        doc = self.pool.get('ir.attachement').search(cr,uid,foo.file_ids)
#        raise osv.except_osv('Object Error', 'Doc %s %s' % str(doc,foo.file_ids))
        if context is None:
          context = {}
        context = context.copy()
        data = self.copy_data(cr, uid, id, default, context)
        new_id = self.create(cr, uid, data, context)
        self.copy_translations(cr, uid, id, new_id, context)
        return new_id
        return super(account_invoice, self).copy(cr, uid, id, default, context)


account_invoice()