You could modify the script to do that, but I think it would be a little weird and get in your way. If you want to manually and explicitly create to tabs that point to the same folder, you should be able to.
You may want to put one in Details and the other in Thumbnails, or use one to start a search while the other stays as it is, or various other things. Making it impossible to have two tabs for the same folder will get in your way more than it will help.
It would also really get in your way if you had, say:
- Tab 1: C:\Windows
- Tab 2: C:\
And you wanted to move Tab 2 to C:\Windows\System32 by double-clicking Windows, then System32.
You would lose tab 1 or tab 2 in the process, even though the desired end result was two tabs in different folders. It just doesn't make sense to do this. Navigating to a folder in a tab that is already open should not close that tab (or any other).
The script could be changed to do what you want, but I'm not interested in making those changes as I don't think it makes sense.
For commands which open a folder in a new tab (not changing what an existing tab shows), you can use the
NEWTAB=findexisting argument to switch to an existing tab.