When this first happened, I though the file had failed to attach. So I
manually attached the file I was trying to send onto the email form.
Then the attachment window popped open to show two copies of the file
attached. I figured that if something was wrong with the attachment
window, I should be able to send myself a file using OO's send-file
function but without manually attaching the file (thus without immediate
visual confirmation that the file was attached). I sent myself a test
file without manually attaching anything, and the file arrived just
fine. So I presume this is an attachment-window glitch in Evolution?