Even with ##RequestID## SDP cloud still create as a new case

Even with ##RequestID## SDP cloud still create as a new case

Hi Team,

I would like to know how to tackle this issue. 
When I created a new case under SDP, system will issued a new ticket number.
I taken the ticket id & even use the  ##RequestID## inside my subject id during i send email via outlook, SDP still record it as a new case. 
                New to ADManager Plus?

                  New to ADSelfService Plus?