To be more explicit, no, you don’t have to scour for plagiarism. In the “purple book”, that is not listed as a “teacher must” (page 76-77 and 82-83 of The Purple Book)
However you must check to make sure your students do not use practice performance tasks from your class (yes, that means they may not use their “Clicker Game” as their Create PT). The Purple Book states that teachers “may not” let students submit practice PTs. I believe students are allowed to make independent modifications and use those, but I think the intention is that students don’t claim as independent work the stuff they created while having the benefits of collaboration, teacher assistance, and feedback (as they likely had with their practice PTs). For example, the way I interpret it, the student can independently add features to their “Clicker Game” and reference those parts for their Create PT, but they cannot claim any part of the original assignment or modifications made as part of the original assignment to be the work they feature for the Create PT.
If you happen to stumble on plagiarism, the College Board instructions are to report it as plagiarism (with evidence, as Terence mentioned), as opposed to returning the work to the student - that is not an allowable reason to return student work.