Webform many dropdownlist results saved under one unique field
Ok, I found a solution: I've used a presave hook. This hook is saving all values submitted by users in a hidden field called "model". A loop will be needed to read the 56 listboxes and save the only value the user have selected.

To do my test I've created a test webform, with only one listbox and one hidden field (this field created automatically my needed field "model" in my database). That speed up a little my tests:

First at all I've created a custom module with a "presave" hook
In a new directory named "duplicated_data", I have create two files:
data_duplicate.info
data_duplicate.module  

In the info file I've declared my module

name = Data Duplicate
description = Duplication Data for Webform
core = 7.x
package = Webform
dependencies[] = webform


In the module file I've written my php code

&lt;?php
function data_duplicate_webform_submission_presave($node, &amp;$submission) {
  $component_id = 1; // dropdownlist cid = 1
  $mem = $submission-&gt;data[$component_id][0]; // retrieve the submitted value
  drupal_set_message('datos :'.$mem); // print the result
  $component_id = 2; // cid = 2
  $submission-&gt;data[$component_id][0] = $mem; // save the data in the hidden field
}
?&gt;


Now I need to activate my custom module.
Each time I make a choice in my dropdownlist/listbox and click on submit, the results will be saved in the database in both fields.

For my production webform , I need to create the loop in order to read all the 56 listbox values and save in my field "model" only the value from the selected listbox. It will be easy to catch it, because it will be the only one, different from NULL.

Hope it will be help somebody else!