How do I omit similar content in a for loop based on the first letter in title? I've got a for loop that is returning the first letter of the entry title. I have a number of entries so I almost get a complete alphabet except that there are too many of many letters. I only want one 'a', one 'b', one 'c' and so 
forth. And no letter if there is no entry to correspond to it.

{% for alpha in craft.entries.section('advertisers').order('title asc') %}
    &lt;a href="{{ siteUrl }}{{ alpha.title|first }}"&gt;{{ alpha.title|first }}&lt;/a&gt;
{% endfor %}


This code returns:
A A A B B B B B C C E E E E G G H H K M M M M P P R R S S S T T U U U W W
but I want it to return A B C E G H K M P R S T U W

Are there any solutions to this?
If you need only unique letters, you can build a new array and only merge first letters that are not already in the array like this:

{% set letterArray = [] %}

{% for alpha in craft.entries.section('advertisers').order('title asc') %}
  {% if alpha.title|first not in letterArray %}
    {% set letterArray = letterArray|merge([alpha.title|first]) %}
  {% endif %}
{% endfor %}

{% for letter in letterArray %}
  &lt;a href="{{ siteUrl }}{{ letter }}"&gt;{{ letter }}&lt;/a&gt;
{% endfor %}