So if your existing TMPL_LOOP section looks like:

<tmpl_loop name="DATA_LOOP">
   <tr>
      <td>
         <tmpl_var name="data">">
      </td>
   </tr>
</tmpl_loop>

Change it to:

<tmpl_loop name="DATA_LOOP">
   <tr>
   <tmpl_loop name="COLUMN_LOOP">
      <td>
         <tmpl_var name="data">">
      </td>
   </tmpl_loop>
   </tr>
</tmpl_loop>

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