Some Gmail users have reported that when they draft an email in Apple Mail, they're seeing duplicate drafts of that email in Astro.
This is actually a known conflict that has existed for years between how Gmail and Apple Mail handle draft messages (not having anything to do with Astro).
You can learn more in this Gmail Help Forum article, "Gmail saving multiple copies of all drafts", or in this StackExchange article, "Apple Mail, producing multiple duplicate drafts on gmail"
Solution, option 1: stop using Apple Mail, continue using Astro :-)
Solution, option 2: turn off autosaving of drafts in Apple Mail on your Mac