I use the localized (German) version of Maildroid so I can’t tell you the exact name of the menu entry. But it’s something akin to “preload and indexing”. There you can choose to never download, download once viewed, or download immediately.
There is also an additional setting on how to handle attachments, depending on size and connection type (mobile or WiFi).
As for the second part I don’t quite understand why there are two copies of the same email in the first place. If an email is present in both inboxes, the combined inbox will show them as two separate emails as it is expected.
