Ok, the reason for this issue makes sense to me now. Basically, during the first upload, the first 64mb segment of sj://testbucket/IMG_1016.MP4 was uploaded, successfully, but the download was either canceled, or unexpectedly failed before the remaining segments could be uploaded. Normally, in the case of a failed download, we would automatically delete the successfully uploaded segments, but there is a bug which we are aware of and working on fixing that causes these “zombie segments” to linger around (Design Draft: Zombie Segments Cleaner).
The reason the second upload worked is because you were using a different filepath, so there is no zombie segment. The original command will fail, just as re-running the new phonemovie.mp4 command will fail. The difference is that the first will fail because of a zombie segment exists which shouldn’t exist, because the entire file was never uploaded to begin with, while the second will fail because a complete, successfully uploaded file already exists.
Unfortunately, this is not something you can fix on your end, but we are aware of the issue and have been working on a fix for it.