"re-completing" topics in a course?
I just completed the third out of six lessons in Travel (https://www.duolingo.com/skill/de/Travel), and now it looks like I have credit for all 6, although I didn't get any extra XP. I've had this happen several times—I think that the lessons got longer from when I first did them more than a year ago, but it's weird for it to show me only partially completing a lesson, and then to give me credit for completing it.
Ordinarily I have gone straight from lesson to lesson, but was looking at badges and saw that I had lessons I thought I had completed that were now partially complete or not complete at all, so I started playing with some of them. It's not a big deal, but it's kind of weird.
That's what happens if you relearn an updated skill. I think it's just a display error.
You had probably completed the Travel skill before an update to the course. After the update, the skill showed as unlearned, but in fact, only a few words were probably added. By learning the third lesson, you learnt what was there to be learned (lessons 4, 5 and 6 just looked locked, probably because you're normally supposed to learn them in order, but there were no new words in them) and the whole skill got unlocked.
At least this is what I think happened to you. I experienced the same when I was relearning my older German skills after an update.