Enable Duplicate Mail Thunderbird 38xxxx

go to

Tools-->Options-->Advanced[some version is General]--Config Editor

[If No have  Menu Bar please  Right Click  on top page  then  Enable Menu Bar]

Type in   Search  "dup"

Search Result is showed

"mail.server.default.dup_action;  user set   integer    0"

Double Click this line

Chang Value  from 0  to  1

press  OK

Close Thunderbird

Open Thunderbird

Finish

*** Now it's permit only one email  copy  in  INBOX****