For example, try to use a function that isn't defined. The error will be displayed twice momentarily, and then the duplicate will disappear after a moment.
It's interesting that the font size on each is different. I don't know why that would be.
This is because the some HTML and text is being created in the comment function of Comments object of the IDE and in the DocumentCommentSpan in Spans.ts. The Spans seems to be the more modern and is the first to appear. When the render function gets called on the IDE object it drills through and calls the render function of the Comments object and injects the new DOM objects as children into the DocumentCommentSpan.
Relevant lines are around IDE and Span