I'm so sorry for making these many duplicate postings.
This is how they were made...
I made a post about pair seating which showed up at once.
Next I tried to make a second post regarding BizJet's posting. At this time I was rejected with a server error.
Next about two hours I kept doing the same second posting while doing some other works, all except the last one were rejected with the same server error.
I didn't do this blindly, though. Every time before I made a post, I reloaded this thread to make sure my rejected posting was not here.

I don't know why, but the server this bbs is on must have been queueing the posting requests without updating this thread and then bursted the queue at once.