Filter list based on their item and get their branch path

I try to re-arrange one of my list according to its content. The tree has 32 branches and 1 item per each branch. The items are exact same strings 16 and 16 times. For further use I would need to gather a new list where I have 2 branches containing 16+16 indexes as the path of the previous branches. Pls see below.

So I am expecting to get a new tree where one branch is containing those indexes from 320 to 335 as they have the same data.

Thank you.