Thank you to everyone who offered advice! Here's how I made it work. I found it easier to do in Apple Numbers, because in Numbers it's easy to add additional tables within a single sheet. I'm sure there are ways to do this in LibreOffice Calc and MS Excel, but I don't how.

I made separate tables for each lecture, consisting of 2 columns each. The "A" column is just a list of all the registered IDs. In the top cell of the second column, I inserted my formula:

SUM(COUNTIFS(Table 1::B,A1,Table 1::E,"初めまして")+COUNTIFS(Table 1::B,A1,Table 1::E,"はじめまして"))

The above example is for the first lecture. I SUMmed two COUNTIFS to account for variations in the password. If there is no variation, I could use just one COUNTIFS and no SUM. I then dragged down the column to apply the formula to the other IDs. When I do this, the number in column "B" should be "1." If it's "0," it means the person didn't submit a comment or they didn't get the password right. If it's "2" or more, that means they submitted multiple comments. Then I do a manual search in the main table to find the extra comment(s) and delete them.

It would be neat if I could have the redundant comments highlighted with Conditional Highlighting, but my understanding of that function in Apple Numbers is that it can't be done with formulae. (I would love to be proven wrong about this.)