W van Cothen wrote:

I see two groups (That is, I see "All groups" plus group X plus
group Y.)

You should see "All Accounts", at a minimum, unless you renamed the default
group to "All groups". Select each group and examine the settings in the
lower half of the window as you do. If you have more than one group
scheduled at the same interval and two of those groups contaion the same
account, then you could see the symptoms you describe.

I hardly dare to tell you, but I suddenly realise that I don't see the
"duplication" anymore. I did not change anything in the settings.
Come to think of it, perhaps it's only now and then that the
phenomenon occurs. A few years ago I had the same thing on my
previous computer but then it was like a fourfold version. The reason
I am asking now is that I think I may be doing silly that causes this
problem.

Perhaps you'll discover the reason.

Do you mean the interval between automatic checkings?
5 minutes.

Ten minutes is a good minimum value. Too short of an interval can cause a
send/receive cycle to begin before the previous one is ended, with
interesting side-effects in the program.
--
Brian Tillman