Would it be good to add a conclusion just to make sure that if you thesis wasn’t clear enough in the beginning its been made more clear. But this conclusion shouldn’t be supplemented or confused with any of the standard things in the DBQ
If you have time after writing your essay to write a conclusion, by all means do it! However, I seriously doubt that people will have the time to write a conclusion.
This year’s DBQ must be written in a very short amount of time, so I would recommend that you focus on your introduction and body paragraphs, because that’s where all the points are earned.
You can earn the thesis point from the conclusion, but it’s much easier to get it in your introduction. Keep in mind that the thesis just has to make a defensible argument about the prompt.
If you have time to write a conclusion, I would use that time to restate your thesis but also to aim for the complexity point by arguing the other side of the historical thinking skill being used: Comparison–tell differences if you told similarities; Causation–tell causes if you told effects; Continuity and Change–if you told how things changed, tell some ways they stayed the same.