If a large agency technical report is published online, would a condensed journal article be considered a duplicate publication? Or would this be similar to publishing a chapter from a thesis as a journal article.
Whether this is duplicate publication likely depends strongly on the nature of the report, the extract, and the journal's policy. Let us set aside journal policy for the moment, and discuss the general principle from a scientific honesty perspective.
One of the key reasons to disapprove of duplicate publication is because it inflates the apparent value of a piece of work to the author (by adding to their CV) without significantly increasing the value to the scientific community (since the article is already out there). So: what is the value that will be added by extracting this portion of the report?
With a thesis, my typical interpretation is that a chapter extract adds the benefits of peer review and condensation of a complex and often sprawling source material into a more accessible form with a clear and focused message. Now let's apply this same analysis to a report: some technical reports have a strong internal community review process before publication, in which case the benefit of peer review is unclear; others are thrown together in a much more hasty and ad hoc fashion, in which case peer review has a much clearer value. Likewise, if your material is already a neatly focused and separable subsection, then the value of an extract is less clear than if it is scattered throughout.
There may be other reasons that an extract publication would be significantly different as well: for example, regulations on the report process might have prohibited certain lines of discussion, or there may be new data that can be added.
The bottom line is this:
- Consider the value added for the scientific community by creating an extract; if it is significant, then creating an extract is reasonable to do.
- In interacting with editors, be completely clear and transparent about what you have done and why you think it's valuable, and while some editors may disagree and reject your paper, you will not face any accusations of improper behavior.