Steps to reproduce:
# Create a simple widget class that extends Y.Widget and augments Y.WidgetStdMod
# In the renderUI function set the bodyContent (via this.set("bodyContent", node))
# Add a logging statement in widget-stdmod.js in the _addStdModContent function (for example: console.log("_addStdModContent"); console.log(node);)
# Run render your widget to a page

Expected behavior: "_addStdModContent" prints once.
Actual behavior: "_addStdModContent" prints twice.

This only happens when bodyContent is set during the renderUI phase. I think the reason this is happening because of the following. _renderUIStdMod listens for the bodyContent change with:

this.after(BodyChange, this._afterBodyChange);

But _syncUIStdMod also checks to see if the bodyContent has changed which it does by checking if _stdModParsed is set. So when bodyContent is set in the subclass's renderUI function, _renderUIStdMod
fires which adds the content to the DOM, but it doesn't set _stdModParsed because _findStdModSection returns null. Therefore when _syncUIStdMod runs during the sync stage, it checks to see if
_stdModParsed is set, which it isn't, so it goes ahead and adds the elements to the DOM again.