I am still using Opera e-mail client (I know it is not supported any more, but I love it and I am still running and old version of Opera basically only for this). In the last 1-2 weeks I have noticed that it duplicates sent email in the sent folder (not in the IMAP "Sent Messages" folder though). I have tried to use MS Outlook and my smartphone but these do not cause the problem, so I guess it is an Opera issue.
A few additional details:

• I am storing email since 2006 on my machine, dozens of Gbs, so reinstalling everythin would be the very last resort
• Our server admin required a password reset about the same time the issue appeared (some google searches for other clients suggested that this may be related to IMAP synchronization...)
• I really love Opera as an email client. I tried to switch to MS Outlook for a few days and I had to switch back to Opera as I lost so much time with standard email management.

Thanks for any help