Hi OmeLuuk / Morpheus,
I can give my side clarification, actually it was my mistake not softwares. I pressed "Submit Reply" and during process indicator, I pressed "Stop" button of my ie for adding smile face, in between my data saved in database, after adding smile face, I again pressed "Submit Reply" result duplicate posts.
Sofware point of view, there should be one lock like -
if (sender = saved sender and subject = saved subject and text = saved text) then
[dont save post]