A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data
Pull the Client Ref into the Rows section, the Date into the Values section and change the aggreation to count.
Set a filter to Count > 1.
If you want to see the dates, load the data into the Data Model and add a measure using CONCATENATEX. https://learn.microsoft.com/en-us/dax/concatenatex-function-dax
Andreas.