FieldChange Script Executes Twice..HELP!!!

I have a custom record which contains  Shipping Qty,  Previous Qty and Qty.

There is a  script that is set to execute on Field Change.

If Shipping Qty is change
{
   "previous qty" is set to "qty"
    "qty" = "qty"-"shipping qty"
}

The problem is that the code gets executed twice.  It is acting as though the shipping qty field is changed twice.  Any ideas why this is happening?  I have similar scripts running on transactions and it works fine, just seems to occur on custom record.

 function updateQtyVal(type, fld)  {   alert(fld);   if(fld == 'custrecord_locqtyshipped')   {    var qtyshipped = nlapiGetFieldValue('custrecord_locqtyshipped');    var qty = nlapiGetFieldValue('custrecord_locqty');                 //Set the previous qty to the current qty on-hand             nlapiSetFieldValue('custrecord_locqtyprev', qty, false);         alert(qtyshipped);       var  newqty = qty - qtyshipped;          if (newqty < 0)      {           newqty = 0;       }             nlapiSetFieldValue('custrecord_locqty', newqty, false);                  //Set the Shipping Date field to today      var sysDate = new Date();      var curDate = (sysDate.getMonth() + 1) + "/" + sysDate.getDate() + "/" + sysDate.getYear();      nlapiSetFieldValue('custrecord_locshipdate', curDate,false);   }    }