KMail 4.7.3 is working in background and fetching Emails from time to time using pop3 and Imap akonadi ressources.
I've got following error message:

Konfliktlösung Kontact
Zwei Aktualisierungen stehen miteinander in Konflikt.
Bitte wählen sie aus, welche angewendet werden soll.
Änderungszeit: 00:00:00 17.11.2011 10:57:20
Flaggen: \SEEN
[[Linke Version wählen]] [[Rechte Version wählen]] [[Beide behalten]]

What is the source of this error message? Headline says "Kontact", but I don't see which Kontact component gives this message.
What does this error message mean?
What is the "left version" or the "right version"?
What is the effect if I choose one of the three buttons?

I have got a similar error message short after changing to KMail 2.

You should be able to scroll up and down to see the two versions and compare them. When I saw such boxes the messages appeared to be the same, so I judged that either duplicates had been created somehow, or maybe there was an html part and plain text part being displayed separately. Anyway, whatever the reason, the body of the messages appeared to be the same, so it didn't matter which one I chose.

Sorry I can't give a definitive answer, but scrolling around should help you make the decision.

It originally was a bug with filtering, whose changes to headers were displayed as differences instead of ignored. It's much less frequent nowadays: nevertheless, if you can trigger it reproducibly, file a bug.