fnRender applied multiple times

Hi,
I wanto to display the same data source column (col1 in the example below) twice in the datatable:
the first time without rendering, while the second times applying a rendering function (renderCol1).

$(document).ready( function() {
       		var dataTableSettingsLocal = {
       			 "bLengthChange": true,	
       			 "bServerSide": true,
                  "sAjaxSource": "list.json",
                  "bProcessing": true,
                  "sPaginationType": "full_numbers",
                  "aoColumns": [
                          { 	"mDataProp": "col1"},
                          { 	"mDataProp": "col2" },
                          { 	"mDataProp": "col1", "fnRender": renderCol1 }                                      
                    ]
                    ...
});

The problem is that I get the function renderCol1 applied to the first colum although I don't call it.
How can I avoid this side-effect?

Thanks