Can I nest a Select Distinct function in a beastmode?

Relatively new to all of this, but I'm working on a beastmode that calculates the average tenure of active employees within my company. The data I'm pulling from includes a lot of duplicate system id's (employee id's). I need to filter out these duplicates in order to get an accurate average. I've written the formula in a myriad of ways--it was valid until I added the select distinct 'system id' syntax. Am I missing something, or is there a workaround I'm not aware of? Thanks for your help.