How is filter hierarchy handled in custom filter?

Hello,

I am trying to create a custom filter which consists of first extracting data from several locations and then combining these extracted locations in a group. Here’s a sort of scheme of what I expect to happen:

Input - Extract Location 1 - Group Datasets (Output)
- Extract Location 2 -^
- Extract Location i -^

When doing this, warnings pop up that the group datasets proxy already exists, so I suspect what actually happens is that for each extracted location a group dataset filter is applied. Something like this:

Input - Extract Location 1 - Group Datasets (Output)
- Extract Location 2 - Group Datasets (Output, replaces previous)
- Extract Location i - Group Datasets (Output, replaces previous)

Is this correct? Is there a way to write a custom filter for what I intend to do?

Thank you in advance!
Davis