Check out this thread and see if this helps solve your issue:

I too have just hit this issue, but am tying my base in knots trying to cater for the ArrayFlatten formatting by adding rollup/lookup fields that don’t appear to be addressing the issue.

Can anyone provide a simple example, where multiple people are being assigned to a task, and in the task summary there’s a field that’s rolling up the names of assigned people - but with duplicates removed. A couple of screenshots would be helpful too.

I was following the thread with the super heroes example, but the solution given didn’t really detail the super hero base and referred to a colours base, and now I’m just confused and I want to be rid of my duplicate multi-select lookups.

Check out these 2 threads and see if they help solve your issue. @Adam_Minich created a video in the first thread:

Thanks @ScottWorld - I suspect I’ll find the method needed within this base too;

Would be great if Airtable Devs could resolve this problem with another aggregation function - it would make life so much easier.