Manual deletion of duplicates typically doesn't work when Sync is involved. Sync daya on the server gets corrupted from a "device" and then "spreads its joy" to other desktop / laptop devices quite easily.

Open Help > Troubleshooting Information and scroll all the way down to Places Database. Then use the Verify Integrity button and allow that process to run to completion and show the results. Best to do that with all desktop / laptop PC's connected to that Sync account; "mobile" devices don't have that Verify feature.

Task: checkCoherence is IMO the important thing in the results.

Then restart Firefox and see if Sync doesn't start doing duplicates again.

Thank you "the-edmeister" for your hint. It seems that your advice has solved the problem

Hello "the-edmeister"! Unfortunately, the problem recurred a few days later, although I implemented your suggestion. Without changing anything, double folders and files suddenly appeared again and again. Do you have an idea how this can be permanently prevented?

The results are: Chronik- und Lesezeichendatenbank Integrität

> Task: checkIntegrity + The places.sqlite database is sane + The favicons.sqlite database is sane > Task: invalidateCaches + The caches have been invalidated > Task: checkCoherence + The database is coherent > Task: expire - The task queue was cleared by an error in another task. > Task: originFrecencyStats - The task queue was cleared by an error in another task. > Task: vacuum - The task queue was cleared by an error in another task. > Task: stats - The task queue was cleared by an error in another task. > Task: _refreshUI - The task queue was cleared by an error in another task.

What else can I do? Thanks in advance for a helpful answer.

You didn't mention what other devices you are using with Sync; nor any mention of whether Firefox for Android (or for iOS) is used on that Firefox Account.

When it comes to the desktop / laptop version of Firefox that Verify Integrity feature has to be used on all those devices at the same time or before connecting any of those devices back to your Firefox Account. Otherwise the corruption just might still be 'making the rounds' and re-corrupting the other devices.

Are you using any other "sync" services or add-ons simultaneously with your use of Firefox Sync?
Known to cause corruption issues in Firefox. One version of "sync"; and never along with Firefox Sync.

After some time with trial and error, the problem now seems to be solved. I guess the corruption was triggered by a connection (synchronisation) between my windows decices and the icloud of my iphone. After deleting it, the sync runs well again. Thank you once more for your advice.