Definition
A manufacturing error, per CGC's own glossary, in which 'two or more covers' are bound to the same comic book due to a malfunction in the offset printing process, typically caught and halted quickly, capping the defect's surviving population at a small number of copies. CGC's glossary explicitly states a double cover is 'not considered a defect' for grading purposes.
In Panel Profits
Panel Profits treats a CGC-certified Double Cover as a distinct, verifiable error-variant class within an issue's Atomic Asset Unit structure — its scarcity (a small, print-run-capped surviving population) supports a separate, materially higher FMV benchmark than the standard base issue, but only once third-party certification has authenticated that both covers share identical print stock and ink.
Comic-market example
A certified Amazing Spider-Man #252 double cover has traded at roughly 4-5x the price of a standard copy in the same grade range, because CGC authentication confirms the error is genuine and the surviving population is a small fraction of the total print run.
Common mistakes
Do not price an uncertified claimed 'double cover' at the certified-error premium — third-party authentication that both covers are genuine, matching print stock is required, since the error is otherwise easy to fake by simply gluing a second cover on. Do not treat a double cover as a grading defect; CGC explicitly does not count it against the numeric grade.