form checkbox validation

First is the subpremier to select.

[CODE]<select name=”SubPremier”>
<option value=””>Select</option>
<option value=”Ashton United”>Ashton United</option>
<option value=”Bradford Park Avenue”>Bradford Park Avenue</option>
<option value=”Burscough”>Burscough</option>
<option value=”Buxton”>Buxton</option>
<option value=”Chasetown”>Chasetown</option>
<option value=”Colwyn Bay”>Colwyn Bay</option>
<option value=”FC Halifax Town”>FC Halifax Town</option>
<option value=”FC United Of Manchester”>FC United Of Manchester</option>
<option value=”Frickley Athletic”>Frickley Athletic</option>
<option value=”Hucknall Town”>Hucknall Town</option>
<option value=”Northwich Victoria”>Northwich Victoria</option>
<option value=”Kendal Town”>Kendal Town</option>
<option value=”Matlock Town”>Matlock Town</option>
<option value=”Marine”>Marine</option>
<option value=”Mickleover Sports”>Mickleover Sports</option>
<option value=”North Ferriby United”>North Ferriby United</option>
<option value=”Nantwich Town”>Nantwich Town</option>
<option value=”Ossett Town”>Ossett Town</option>
<option value=”Retford United”>Retford United</option>
<option value=”Stocksbridge Park Steels”>Stocksbridge Park Steels</option>
<option value=”Whitby Town”>Whitby Town</option>
<option value=”Worksop Town”>Worksop Town</option>
</select>[/CODE]

them the main selection which they have to select only 3. I have a check to allow only 3.

[CODE]<div id=”div1″>
<INPUT type=checkbox value=”Ashton United” name=”premier1″>Ashton United<BR>
<INPUT type=checkbox value=”Bradford Park Avenue” name=”premier2″>Bradford Park Avenue<BR>
<INPUT type=checkbox value=”Burscough” name=”premier3″>Burscough<BR>
<INPUT type=checkbox value=”Buxton” name=”premier4″>Buxton<BR>
<INPUT type=checkbox value=”Chasetown” name=”premier5″>Chasetown<BR>
<INPUT type=checkbox value=”Colwyn Bay” name=”premier6″>Colwyn bay<BR>
<INPUT type=checkbox value=”FC Halifax Town” name=”premier7″>FC Halifax Town<BR>
<INPUT type=checkbox value=”FC United Of Manchester” name=”premier8″>FC United Of Manchester<BR>
<INPUT type=checkbox value=”Frickley Athletic” name=”premier9″>Frickley Athletic<BR>
<INPUT type=checkbox value=”Hucknall Town” name=”premier10″>Hucknall Town<BR>
<INPUT type=checkbox value=”Northwich Victoria” name=”premier11″>Northwich Victoria<BR>
<INPUT type=checkbox value=”Kendal Town” name=”premier12″>Kendal Town<BR>
<INPUT type=checkbox value=”Matlock Town” name=”premier13″>Matlock Town<BR>
<INPUT type=checkbox value=”Marine” name=”premier14″>Marine<BR>
<INPUT type=checkbox value=”Mickleover Sports” name=”premier15″>Mickleover Sports<BR>
<INPUT type=checkbox value=”North Ferriby United” name=”premier16″>North Ferriby United<BR>
<INPUT type=checkbox value=”Nantwich Town” name=”premier17″>Nantwich Town<BR>
<INPUT type=checkbox value=”Ossett Town” name=”premier18″>Ossett Town<BR>
<INPUT type=checkbox value=”Retford United” name=”premier19″>Retford United<BR>
<INPUT type=checkbox value=”Stocksbridge Park Steels” name=”premier20″>Stocksbridge Park Steels<BR>
<INPUT type=checkbox value=”Whitby Town” name=”premier21″>Whitby Town<BR>
<INPUT type=checkbox value=”Worksop Town” name=”premier22″>Worksop Town<BR>
</div>[/CODE]

How do i check that they have not selected the same team again in the second checkbox. need an alert to say they have please.

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