Please follow below steps to download your certificate (worked for me!)
1. Login to Splunk support portal with your registered id.
2. From home page, navigate to My Certifications link at left side under highlighted "welcome User"
3. Click on that link and you will be taken to the page where you can see all certifications you have completed.
4. Downland one which you would like to have a copy.
Hope I answered your query!!
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