I've made the following modification to civicrm/templates/CRM/UF/Form/Block.tpl (see below) that allows the primarily email field to be specified in the profile and displayed - without a duplicate as is the case in the demo @

http://demo.civicrm.org/drupal/civicrm/event/register?id=15&reset=1

Block.tpl:

{if $n eq 'email-Primary'}
            {assign var=n value=email-$bltID}
            <td class="label">{$form.$n.label}</td>
            <td>{$form.$n.html}</td>
        {else}             
      
               <td class="label">{$form.$n.label}</td>
           <td>
                 {if $n|substr:0:2 eq 'im'}
                   {assign var="provider" value=$n|cat:"-provider_id"}
                   {$form.$provider.html}&nbsp;
                 {/if}
                 {$form.$n.html}       
           
        {/if}

With the change made, the email field entry/display that is present in other templates can be commented out. ie.

{*
{assign var=n value=email-$bltID}
<table class="form-layout-compressed">
    <tr><td class="label nowrap">{$form.$n.label}</td><td>{$form.$n.html}</td></tr>
</table>
  
    *}

Attached are the files that I modified. Hope it's useful to everyone else