Could anyone please tell me, is this inability to create a set from a list of planes an intended behaviour?
Because the workarounds I came up with seem a bit redundant:
Create Set works with data types that are not text. It doesn’t work with geometry though.
I think the components in the Sets tab are mostly thought to be used with text.
Why don’t you use Cull Duplicates?
cull-duplicate-planes.gh (17.4 KB)
Is that what he was trying to do? 
Thank you, this will work.
Thank you. I didn’t know this.
Oh. It fails the moment there is no duplicated data.
Please share your file if you persist to have problems.
List Length with the expression * = x-1, cast to a boolean and use as the Gate input of a Stream Filter component.