Having thousands of puzzles to choose from, the puzzles to solve in the training section should be unique. However, https://en.lichess.org/training/61042 came up for the third or forth time today, and I've only done a small number of them so far.
AFAIK you get a puzzle again if you didn't solve it correctly, so that you actually learn from your past mistakes.
Is this really the intention? I understand the idea to show a puzzle again some time, but imho this is done way too soon and really too often, so I consider it a bug on an otherwise great site.
@Vetinari_Computer is correct. When the tactics trainer was updated a few months back, one of the things the developers implemented was spaced repetition. You can read about how it works here: https://en.lichess.org/blog/WDY6cCEAALYi5Xg2/puzzles-update.For more about spaced repetition, see here: https://en.wikipedia.org/wiki/Spaced_repetition
I still would like the repeated puzzles to not be rated.