Some mail services, like Outlook, already automatically copy sent mail to the Sent folder. When Mailspring sends a message, it does the exact same thing, resulting in multiple copies of the message.
This replication gets worse with multiple recipients, including in BCC fields. This half of the problem is related to read receipts. (See BCC'ing many recipients splits the message into one for each recipient · Issue #1669 · Foundry376/Mailspring · GitHub)
However, the message still is only being sent once.
Odd related bug:
I can confirm this bug on Mailspring 1.8.0 and Ubuntu 20.04. For me, this only occurs with Outlook. None of my IMAP accounts have this problem.
It’s useful to note that Evolution had the same problem, which I mitigated by not specifying a Sent Mail folder.
This would be easily resolved with a per-account option for “Copy outgoing messages to send mail”.