When you say you noticed yesterday evening, around what time? If it was after midnight, then it was likely due to an issue when your connection closed last night while we rebooted the cluster for maintenance.
No, before midnight.
Messages duplicating like that whould be due to either the client for some reason duplicating them, or an issue with the index files on our side. To rule out the latter, I purged your indexes, which will cause them to be rebuilt the next time you connect.
To be clear, the messages were not duplicated, they were just out of order with respect to arrival time. Your purge of the indexes did make a difference. The messages from 2019 that were mixed in with today's messages are no longer appearing there in my message list. A single message from 1-1-2018, which may be the message that should be first in the list, appeared near the end of my message list, just before the message telling me about your post here.
An issue with the index files may have caused the other problem, but that wouldn't have been my first thought. Let me know if clearing the indexes changed anything for you.
When I attempt to look at the list of messages starting from 1 in the index, I only get part of a screenful and then the IMAP connection closes. First time about 5 messages, then I reconnected and got 10, then most recently 35. So the index seems to build slowly, and the activity of building the index may have some effect on the connection closing.