useless since duplicate characters are added clicking on proposals or even backspace
I had the same problem on the Google website. I did not see the problem on Bing, so I think Google Instant Search might be triggering the SwiftKey auto complete on each keystroke including backspace. Not sure if and how my experience is related to your experience in Polaris, but the symptoms are the same.
I experienced the same problem. I use Polaris Office. I found hitting the backspace aded letters and then did delete some but not all of the letters.