I wasn't interested in two weekly challenges I received this week: trial and battleground. So I re-rolled both, and ended up with this:

First I re-rolled the trial. Got the boss one. Then I re-rolled the battleground. Got the boss one. If my math is right, there's only a 0.16% chance of that happening, but it's been a while since I've tried calculating something like that, so I could definitely be wrong (1/25 * 1/25 = 0.0016).
Sure, I could have kept them and had an easy week, but I want enough stuff to do, so I prefer unique ones. It took me four re-rolls to get an acceptable set, even though there were only two challenges I didn't like (1 for trial, 1 for battleground, 2 for the duplicate boss ones).
The latest developer update says, "Our goal is to provide additional ways to customize the experience for progressing the Tome, whether you reroll for Challenges that fit your playstyle more or you want to try for duplicate Challenges you can progress simultaneously." Okay, so re-rolling duplicates will continue. It's a feature.
Can we have a setting that disallows duplicates, so that players who want a unique set every week won't have to waste re-rolls and gold on duplicates?