If I am already a Salesforce Certified Platform App Builder, do I need to take the Salesforce Certified Force.com Developer - Summer '16 Release Exam?
As you may be aware, In 2015 Salesforce had introduced newer certifications for Developer and Advanced Developers, which now is App builder and Platform Developer 1 & 2.
Quote from Salesforce university:
Our Force.com credentials will begin retiring in July of 2017, so choose the best transition option for you. The retirement process for these credentials will be complete as of March 30, 2018.
It means the developer certification release exam is still valid until the certifications (401 and 501) are officially retired. So maybe after July 17 (Winter '17), it would be removed from webassessor as well.