Drop Down List appears twice if user double-clicks selection?

Question

This page is dynamically created and the control is just a generic HTML drop down control. Sometimes if you double click certain items in the list (usually the middle options are more problematic than the others) it'll display the list of items twice, like so:

I've tried to Google around to see if this was a known issue with either .NET, IE or what but I cannot find a single relevant result.

Has anyone seen anything like this before? I'm completely at a loss as to what might be causing it. Using IE11 in Compatability Mode.

Looking at the ASPX page that gets generated in Visual Studio the control looks like this:

<div id="dlRepOpt_ctl02_DynControl">
    <SELECT NAME='Aedates' Class='clsComboBox110' onchange='mDataChanged=1;'>
        <option value='Previous Month'>Previous Month</option>
        <option value='Current Month'>Current Month</option>
        <option value='Previous Quarter'>Previous Quarter</option>
        <option value='Current Quarter'>Current Quarter</option>
        <option value='Previous Calendar Year'>Previous Calendar Year</option>
        <option value='Current Calendar Year'>Current Calendar Year</option>
        <option value='Last 12 Months'>Last 12 Months</option>
        <option value='User-defined Date Range' Selected>User-defined Date Range</option>
    </SELECT>
</div>

Nothing crazy.