Codeigniter on duplicate key insert - php

Question

i want to create a condition which a user would choose whether he wants to use an input which means a database would create an auto_incremented id or he wants to use an older data which will not use new id but only use it. i have used dropdown database populate for my old input.
My Dropdown list
//get contractor list
$Contractor_List = $this->foo_pro->get_list_contractors();
$opt = array('' => '');
foreach ($Contractor_List as $Contractor_No) {
$opt[$Contractor_No] = $Contractor_No;
}
$data['con_list'] = form_dropdown('',$opt,'','ProjectID = "Contractor_No" name="contractorNo" id="" class="w3-select w3-border w3-hover-light-grey"');
My Controller
public function save_createdProject()
{
$data_project = array();
$data_contractor = array();
//project data
if ($this->input->post('year') === '') {
$data_project['P_Year'] = 15;
}else{
$data_project['P_Year'] = $this->input->post('year');
}
if ($this->input->post('code') === '') {
$data_project['Code'] = 'KO';
}else{
$data_project['Code'] = $this->input->post('code');
}
$data_project['ProjectID'] = $this->input->post('project');
$data_project['Contract_Amount'] = $this->input->post('camount');
//contractor data
if ($this->input->post('cname') === '' && $this->input->post('caddress') === '') {
$data_project['Contractor_No'] = $this->input->post('contractorNo');
}else{
$data_contractor['Contractor_Name'] = $this->input->post('cname');
$data_contractor['Contractor_Address'] = $this->input->post('caddress');
}
$this->foo_pro->add_project($data_project, $data_contractor);
redirect('Main/project','refresh');
//var_dump($data);exit;
}
My Model
public function add_project($data_project, $data_contractor)
{
//contractor
$this->db->insert('contractor', $data_contractor);
$Contractor_No = $this->db->insert_id();
//project
$data_project['Contractor_No'] = $Contractor_No;
$this->db->insert('project', $data_project);
$ProjectID = $this->db->insert_id();
}
this here is my problem except inserting the older data which is the contractor_no 1 it creates another data that would insert as contractor_no 4.. but also i need to insert new data for new contractor_name..