I have recently been having a lot of problems with what I think might be a bug in the new update. It seems like almost every time I move a part, there will be another one in the same place; for example, after I drag a gas tank into the sandbox and try to move it, it will move but there will be another one there, and when I move that one, there will be still another there!
At first I thought it may just be a problem with the aircraft mirroring, but it still happens even on a new aircraft where I haven't mirrored anything. If you are experiencing the same problem or have a way to fix it, please let me know. Thanks!