NGLView widgets showing up twice

[lilyminium]

Describe the bug
NGLView widgets are showing up twice in the HTML conversions from Jupyter notebooks. This happens on the Travis-deployed user guide so it's not just my local environment.

Expected behavior
It should only show up once, as there is only one widget output in the notebook.

Desktop (please complete the following information):

• Browser: Safari, Chrome, and Firefox -- browser agnostic as the widget is in the HTML

Additional context
The widgets aren't linked, so they're not two renderings of the same object. Not sure when this started happening, but I don't think the widgets were doubling up when I made the density analysis notebook on February 20. Normal ipywidgets are also created twice. Basic nbconvert to HTML does not result in this issue.