I've stumbled upon a bug which causes additional station for the Train Supply Manager priorities to be created when I select a template in Train Scaling.
A history of the bug can be found in this thread: https://mods.factorio.com/mod/train-pubsub/discussion/5c2210aeda5537000bf302f9

The save can be found at: https://www.dropbox.com/s/2bpgk5r9ng3nvmd/Train%20Scaling-TSM%20Bug.zip?dl=0

Reproduction:
I have multiple priorities in TSM
I select any of my stations that have at least one template available, but scaling not yet enabled
* As soon as I select the template for a station a station gets added to the priorities of TSM

If you need any more information feel free to contact me.

I'll dig into this, thanks! Can you provide a little more detail on what specific actions I need to take to see the bug in TSM? I haven't used it before and I'm going to need some time to digest this big ol' PDF on how to use it :)

Thanks for the response. I've created a short video on how to reproduce in the uploaded save: https://imgur.com/a/aFJGoO3
Basically you look at the priorities of the TSM with the yellow upward arrow and see that each one has exactly one station.
Next thing you do is enable scaling on any station or disable it and enable it again.
When this is complete one priority will have a second station assigned.

The thing that causes the bug is the template selection. Other action such as choosing the scaling station to not trigger the bug.

Using the drop down button for "selecting the train which will be used as the template for this stop" results in firing an event in TSM which is normally only fired by on_gui_click event with a specifically named gui element. Changing the name of the gui element within tsm has no effect - the error condition is still created.

I dropped a note in the TSM bug report - the issue appears to be in TSM's on_gui_selection_state_changed handler - it's acting on changes to all dropdowns not just the ones TSM owns. Should be a relatively painless fix!

Thanks yes - I think this does fix it.

Thanks for looking into the report. Fixed with latest version of TSM