Pressing DEL frequently deletes two e-mails; only one restores w/ CTRL-Z

This might help you. See thread below.

I see on the EMC thread below, two peeps that had the issue turned out to be a “Specific Logitech type mouse” that had a known issue. See end of the thread below, apart from “Conversations” enabled etc.

when deleting, two messages are deleted - eM Client