Great to know about the copy function in the web interface. Doing some cursory reading -- and someone correct me if I'm wrong -- I think if your Fastmail account is configured to use folders, you can only copy an email to multiple folders when using a third-party client that supports the functionality, however, if you enable labels, you can assign multiple labels to a single email in the web client and have that email be reflected as copied in folders of the same label name in any third-party client.
https://www.fastmail.help/hc/en-us/articles/1500000278282-La...