This post doesn't fix what I need: Get all non-unique values (i.e.: duplicate/more than one occurrence) in an array

I'm working with a control that will fail if duplicate keys are found.

I'm trying to obtain a list of duplicates keys and then update the list to remove the duplicates and return that list so the control loads.

How can I do this in JavaScript? I've tried to use includes() but it isn't working.

So the result would be:

8 items now

and Braves and Dodgers would be in another list as the duplicate values.

1. Braves
2. Reds
3. Braves
4. Dodgers
5. Yankees
6. Dodgers
7. Rays
8. Tigers
9. Indians
10. Astros