The Message->Re-edit command allows the user to open a new copy of a sent message so that it can be edited and sent again.  The copy preserves the headers from the original message, which is generally a good thing.  But one header it should *not* preserve is the Message-ID field, as this violates the uniqueness requirements of RFC 2392, RFC 5322, etc., and in turn can interfere with the storage, retrieval, and presentation of messages in mail clients and other mail processing software.

For example, in Thread View, Claws Mail itself gets confused by duplicate Message-IDs, anchoring the reply tree at only one of the messages.  (This is particularly noticeable when using "Re-edit" to send messages to a mailing list that you subscribe to.)  More seriously, having substantially different messages with duplicate Message-IDs can lead to data loss when running Claws Mail's duplicate filter, which checks only the Message-ID header and not the message content.

Steps to reproduce:
1. Navigate to a "Sent" folder and select a message.
2. Message->Re-edit
3. Edit the message content and/or headers
4. Message->Send
5. Using View->"Show All Headers" or View->"Message Source", examine the "Message-ID" header of the original and "re-edited" messages.

Observed behaviour:
6. The Message-IDs are the same.

Expected behaviour:

6. The Message-ID for the "re-edited" message should be different.