This is a new feature in TA version 5.0.17 and up which prevents you from uploading a file with the same filename twice. This is to help prevent accidentally uploading the same file a second time. TA stores the file names of every file ever uploaded, so you cannot reuse a filename even if the file was previously uploaded on a version before 5.0.17.

Solution is to just change the filename to be unique. A common way of doing this is adding a datestamp to the end, like test100915.pb4.