Apple Mail, producing multiple duplicate drafts on gmail
I've seen this behavior using Apple Mail since macOS Sierra. I haven't found a workaround d yet, but I know the email is sent because there is a copy in my Sent folder. Once a week or so, I go into the Google Inbox (should work for classic GMail too) Drafts folder and delete the email threads that are marked "Draft". This clears up the duplicates from Apple Mail on next sync. 

My guess is Apple Mail, when saving drafts periodically to the server (Google) while one is drafting an email, is not using the right mechanism (API call) such that every draft version collapses into one on Google end. This incorrect mechanism results in each draft showing up as individual "drafts".