I bought eClient, a very good program as an alternative to Outlook. Unfortunately, emails (imap) are now arriving in double and triple.

How can I prevent this.

I have three email accounts: Greetings Jürgen

Did you perhaps setup message forwarding on your server (through the email web interface)?

Best solution as a Pro License user is to take advantage of your VIP Support option and open a support ticket with eM Client. They will be able to help you further, either by email, or through a remote connection using TeamViewer.