I still need to clean it up a bit to make it more usable. However, as of now you need to:

1. Export the current tree from Tabs Outliner (can be found on the Backup page in Settings).

2. Place the exported tree in the same directory as the script.

3. Change the name of the "export_file" variable in the script.

4. Run "python dedupe.py".

5. View the modified tree export file in Tabs Outliner (right under the button export the tree).

6. Optionally, delete all the nodes from the current Tabs Outliner session.

7. Copy over the root node to the Tabs Outliner tree for the current session.

I know this topic is super old, but I created a super simple script to dedupe an exported tree backup. You can see it here: https://gist.github.com/jalaziz/03ecd04e44d3fc8bc393448c04e580ef