iPad - Delete email from 1 account, same e-mail deleted from another

I have 2 IMAP accounts set up on my iPad, one for me (dan@xxx.com), one for my wife (sue@xxx.com).

If someone sends the same e-mail to both of these accounts (i.e. both accounts in the "To:" field), then when I look at it, say under the neil@xxx.com account and then delete it, it also gets deleted on my wife's account (sue@xxx.com), so when she gets her account up it isn't there.

I disagree with previous posts commenting 'that's how mail is supposed to work'. Also, this does not happen when viewing/deleting on iPhones or Mac (or browser for that matter).

All threading is turned off on all devices.

any ideas?

thanks