Just for completeness, the root cause of the issue was:
We make a reload call based on certain conditions, and that sets the URL for the combobox. On load of the new JSON data for the drop down, once the URL is set the combobox makes another URL call for data using the cached URL value.
e.g.
$('#' + type + 'InitiatorThreshold').combobox('reload', '/controls/ajaxBankUsers.aspx?a=getThresholdListPlus&r=' + $('#Account').combobox('getValue') + '&i=' + $('#' + type + 'ThresholdId').val() + '&q=' + missingID);
So every time we load JSON data we need to set the URL back to null:
$('#' + trans[x] + 'InitiatorThreshold').combobox({ data: data[trans[x]].Thresholds, url: null });
(Thanks to your email support team. :-)
P.S. Not sure why both execute. I would think that when I load a JSON object for the data, that the URL wouldn't execute. But now that we know, we can avoid it.