I use delay send so after composing a new email and clicking Send, the message goes to my Outbox for a few minutes before sending. If I decide I want to edit the message before it sends, I’ll double click on it to open it and make my edits. While editing, I notice the message the original content sends while I’m still editing it. Then I click Send on the modified message, and it sends too. So the receipts gets two emails from me, the original and then the modified message. EmClient should not send a message if its opened for editing.
You want to either move the message from Outbox to Drafts if you intend to edit it, or delete it from Outbox to prevent it from being sent.