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A guide to help students and faculty understand and avoid plagiarism.
If you can find the same information in multiple places, stated in relatively the same way, it's common knowledge. (Generally, it is said that you should find the information in three to five sources.)
If most people are aware of this fact, or if it's general reference, it's common knowledge.
Sometimes it's a judgment call because what seems like common knowledge to one person isn't to another.
CAUTION: Opinions and unique terminology/phrasing do not qualify as common knowledge.