Clear Certain Fields!!!!!!!

hi,
im using the following code to loop thourgh field to prevent same value being added twice by the user in different fields,

function ClearPart(){
var i = 0;
for (var i=0;i<letterArray.length;i++){
eval('document.Hours.'+ letterArray[i] + '1' +'.value=""'
)
}
}

function checkpart(a){
for (var i=0;i<letterArray.length;i++){
val = eval('document.Hours.' + letterArray[i] +'1')
if (a.value==val.value && a!==val){
eval('a.value' + 'value=""')
alert( "Part number entered twice." );
ClearPart()
break;
}
}
}

how can i change clear part so that once the fields withsimilar values are found, either both the duplicate fields or one with same value are cleared or the second one to be entered the same value in, is cleared,
help plz
thank you