Running a function only once per unique value

Hello, for a report I am building I need to get the reporting structure of each user in the report. I have a list of user emails like this

And I have a custom function I run that calls an API that gets the users management structure. This works great but currently this custom function runs on every single row, even if I have already run it for a user

= Table.AddColumn(#"Changed Type3", "SVP", each try fnGetSVP([AssignedTo.UserEmail]) otherwise "")

This is a large table which means I'm making tens of thousands of API calls, even though there are only ~700 unique users in the table, so the vast majority of those API calls are duplicates which drastically increases the the time it takes the report to run. I cannot use the built in "Remove Duplicates" function because I need to preserve every row for the final report, and it doesnt look like there is a good way to remove duplicates and preserve each row. 

So what I'm looking for is a way to run the function only on unique values. I've tried a few different things, such as extracting the column, removing duplicates, running the function, then creating a one to many relationship back with my original query but that didn't work. If there is a way to do this I'd appreciate the intel!