@peridoc: Thanks for the test. I've emailed you couple of intermediate builds, to further narrow down the problem. Thanks in advance.
After some incremental version testing, I can say that it looks like the issue happened when going from version 6.6.1 beta to 6.6.2 RC. We are not seeing the issue happen in version 6.6.1 beta, but it does occur in versions after that.
As for the file size issue, we are really able to test with small HTML files and this is where we are able to replicate the issue. I don't really have the ability to test "large" files at this time.
Let me know if you need more info. Thank you!
Testing the intermediate versions sounds like a good way to narrow this down. It would also help to check whether the duplication happens with both small and large files. That could give a clearer clue about what changed in 6.7.
I upgraded from v6.5.7 to version 6.7 and I have seen multiple instances where the final portion of text or HTML documents is duplicated (for example the closing "</html>" tag or a portion of it is repeated – where it does not appear in the source local document, but WinSCP adds this after a transfer occurs on the remote site causing the text duplication). This happens via the GUI and with both SFTP and FTPS (implicit SSL). I downgraded to version 6.5.7 and I no longer see this text duplication when completing transfers in my testing. I just wanted to report this as I had not seen it mentioned previously. Thank you!