<input type="text" [B][COLOR="#FF0000"]id="min10"[/COLOR][/B] name="qty[]" size="2" maxlength="2" value="" onchange="check_qty();" autocomplete='off'/>
The probable source of your trouble is displayed in red. You reuse that same ID (and others) on the same page. That's not allowed; each ID must be unique.
Another way of coding the script may be:
<input type="text" name="qty[]" size="2" maxlength="2" value="" onchange="check_qty(this);" autocomplete='off'/>
and:
// Quantity Check - 10 Lengths
function check_qty(el) {
var qty=el.value;
if(qty<10) {
alert("Sorry but this product has a minimum order of 10 lengths. Please enter 10 or more in the quantity box.");
el.value='';
}
}
I didn't test this so "adjustment" may be necessary... 
EDIT: Padonak beat me to it, but only barely 