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There is no check when bying the voucher, and I can't remember that there is a check when you plan for your exam. However you won't be certified if you haven't got your prior certification requirement.
The answer below is about mandatory courses for the master certifications, but I guess this rule also applies for pre-requisite exams.
From Oracle University Frequently Asked Questions
Do I need to complete all the tests in my certification path within a set period of time or in a specific order?
Requirements for Oracle Certifications do not need to be completed in a specific order. It is recommended that a candidate complete the training prior to sitting for an exam, but it is not required. The Course Submission Form should be submitted only after the training has been completed.
I am already sun certified java programmer (SCJP). Now i will be giving web services exam in few days. I have purchased the voucher for the same from PearsonVue center and you are right it does not check at the time of purchase of the voucher.
You are also saying : "you won't be certified if you haven't got your prior certification requirement."
But my question is how oracle is going to validate this if i am already java certified(as java programmer certification is required for web services certification) or not???