I believe the “evidence” he listed could either be obtained simply by looking at the number of solves, or was completely useless information. For example:

• ***had a large performance gap between contestants / some problems were hard / some were easy. (The problems were already ordered by difficulty & This is just a personal judgment and should not be taken as a reliable reference and you could also see the number of solves, so was this useful at all?)
• *** did not solve problem ***. (Was this useful in any way? Could it help anyone solve or pass a problem?)
• The scoreboard was not misleading. (Was this useful? And couldn’t you tell that just by looking at the scoreboard?)
• There bugs on CF. (Was this useful?)

If you participated in the contest, I believe nothing I said would help you solve any problem. If you did not participate, then it was even more useless.

It was just an online contest. As long as you did nothing unfair, such as cheating or revealing solutions, I think reasonable discussion should be allowed(for example, saying that *** was rank 1 or that *** solved problem ***), since all of this information was public.

As for the reused problem, we indeed did not find any similar problems on CPRet or yuantiji, but during the contest, someone pointed out that 1F had previously appeared in a research paper.

We sincerely apologize for this. The duplicate happened because our plagiarism checking was not thorough enough. However, since the problem has already been used and I believe it is a high-quality and educational problem—perhaps even the only useful problem involving this particular concept—I think it would be better to enjoy the problem rather than continue complaining about it.

You can downvote this blog if you want.

By the way, Codeforces does seem to have a display bug: under certain conditions, some users may see an incorrect problem statement, which has been confirmed to be an older version. I sincerely apologize if this affected your attempt, but I do not believe this was my fault.

And can anyone recommend some good methods for checking whether a problem has appeared before?

We couldn’t find 1F using CPRet or yuantiji, because its possible sources seem to be:

1. A Tsinghua University assignment from about ten years ago;
2. A Codeforces blog post from at least eight years ago.

I’m not sure how we could have checked for duplicates in a situation like this. Does anyone have a good solution?