The first thing to check is in Menu > Tools > Settings > Mail > Sent.
Thanks for your reply. This had been done, however when ‘conversation’ mode is turned on, a copy is not saved in sent. I have had to turn off ‘conversation’ mode so a copy appears in the sent folder.
The message is still in Sent, but because it is part of the conversation, it appears to be also in Inbox. The same conversation should be in both folders.