I also failed to reproduce the issue. But I'll sum up what I found out so far:

• wikiEd adds “another” summary line hidden underneath the actual summary line, to offer a convenient method to choose from a preset of summaries. That’s Especially helpful for recurring tasks.
• That HTML select field is behind the text field for the summary, but can be expanded using the arrow down button on the right. Whatever is selected then in the list of items is automatically copied into the text input.
• I could not reproduce the issue, but it seems that sometimes the correct placement of these select input box fails and the user now sees “two” summary lines. There’s an older visualization of this: File:Duplicate summary line.jpg
• Since it occurs randomly, it’s probably a race condition somewhere, so that when the page is not fully rendered, the calculation on how to position the select field fails.

Looking at the code, there's only one thing that seemed to me as a possible point of failure and that's when the width for the summary fields is set. ( See`wikEd.ResizeComboInput()` ). If something's wrong there, because the parts of the interface are not loaded yet, that are needed for the right calculation the result could look like

That's a different issue as in the screenshot above though.