save as --- looping in purge-save-cancel dialog

[Havanna]

In many years of erratic PCs I acquired the habit of frequently saving my stuff in a new file.

When I do this in Livecode and save my current file "zing_028.livecode" via File -> Save as to "zing_029.livecode" I ( sometimes, but not always) get this dialog:
"A stack with the same name as the one you are trying to load is already open" with options to save, purge or cancel the newly saved stack.
This then jumps back to the older Stack - I would want to go on with the newer one.
Oky so far, chuck it out of memory and open the new one. But sometimes that dialog keeps coming up again and again, so I have to kill Livecode and restart.

Where am I going wrong?