Just for you, I went through my logs for my current irc server and checked how often irc messages occured on the same millisecond. I found on the order of a few thousand such events out of millions of lines total.

In addition, that's just based on my perception; IRC is NOT an ordered protocol, so if multiple people ran this same logger, they'd end up with different stream and different duplicate keys (same thing as events lost I might add).

My log, also, conveniently contains joins, parts, notices, ctcps, and all that other jazz because I used a mature logging library.