Hi @Louis63 ,
Please try this code to create a calculated column.
Turnover Generated =
DISTINCTCOUNT ( 'FACT_Sales 2019-2021'[Turnover] ),
'FACT_Sales 2019-2021'[Customer Key] = EARLIER ( 'TableName'[Customer Key] )
If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.