Email sent from non-GMail account to GMail appears as duplicates in (1) Sent Folder of **GMail** (as well as in (2) Sent Folder of the sending non-GMail account, as expected) and (3) GMail Inbox (as expected). Another facet of the same problem is the following. Email sent from GMail to the non-GMail account shows up in (1) Sent Folder of GMail (as expected), (2) **GMail** Inbox (NOT expected) and (3) non-GMail Inbox (as expected). P.S. (1) All of the sending and reception are done via Thunderbird 52.2.1 (64-bit) on Xubuntu 16.04 LTS. (2) Since GMail automatically saves outgoing items in its Sent Folder, I have disabled saving GMail sent copies from Thunderbird.