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. 

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