Why does xul overlay create 2 ids, instead of combining?

I'm seeing this in the browser toolbox:

I would think it would combine my overlay with the same id - why does it create two with the same ID?

<?xml version="1.0" encoding="utf-8"?>
<overlay id="dtInspectorOverlay" xmlns="http://ift.tt/ZJ3Hg8">
<popupset id="inspectorPopupSet">
    <menupopup id="inspector-node-popup">
    <!--    <menuitem id="node-menu-edititem"
            label="Edit HTML"
            accesskey="E"
            oncommand="dt.editHtml()"
            position="1"/>-->
        <menuseparator position="2"/>
        <menuitem id="node-menu-setA"
            label="Set Node A"
            accesskey="A"
            oncommand="dt.setA()"
            position="3"/>
        <menuitem id="node-menu-setB"
            label="Set Node B and Compare"
            accesskey="B"
            oncommand="dt.setB()"
            position="4"/>
        <menuseparator position="5"/>

    </menupopup>
    <menupopup id="rule-view-context-menu">
      <!--<menuitem label="Copy Selection" accesskey="C" oncommand="dt.copySelection()"/>-->
      <menuitem position="4" id="dtCSSCOPYURL" label="Copy URL" accesskey="U" oncommand="dt.urlCopy()" />
      <menuitem position="5" id="dtCSSOPENURL" label="Open URL in New Tab" accesskey="O" oncommand="dt.urlOpen()"/>
      <!--<menuitem label="New Rule..." accesskey="N" oncommand="dt.newRule()"/>-->
    </menupopup>
</popupset>

<script src="chrome://devtooltweaks/content/lib/FlexiColorPicker.js"></script>
<script src="chrome://devtooltweaks/content/inspectorTweaks.js"></script>
</overlay>