9.0: Update PR_REVIEW.md for code duplication

PR_REVIEW.md

@@ -27,6 +27,7 @@ General gulp commands include separate commands for serving the codebase on a bu
 - Verify no code quality violations are present from linting (should be reported in terminal)
 - Make sure the code is not setting cookies or localstorage directly -- it must use the `StorageManager`.
 - Review for obvious errors or bad coding practice / use best judgement here.
+- Don't allow needless code duplication with other js files; require both files import common code.
 - If the change is a new feature / change to core prebid.js - review the change with a Tech Lead on the project and make sure they agree with the nature of change.
 - If the change results in needing updates to docs (such as public API change, module interface etc), add a label for "needs docs" and inform the submitter they must submit a docs PR to update the appropriate area of Prebid.org **before the PR can merge**. Help them with finding where the docs are located on prebid.org if needed. 
 - If all above is good, add a `LGTM` comment and, if the change is in PBS-core or is an important module like the prebidServerBidAdapter, request 1 additional core member to review.