What is double patenting in patent law?

Double patenting is a doctrine in U.S. patent law that prevents a single applicant (or a commonly owned set of applications) from obtaining two patents covering either identical or not patentably distinct inventions. There are two distinct types: statutory double patenting and nonstatutory (obviousness-type) double patenting. Each has a different legal basis, a different test, and a different set of remedies.

Statutory double patenting

Statutory double patenting arises from 35 U.S.C. § 101, which states that a qualifying inventor "may obtain a patent," with the singular article "a" interpreted as limiting an inventor to one patent per invention. The rejection applies when two claims cover identical subject matter.

The diagnostic test: could an embodiment literally infringe one claim without infringing the corresponding claim in the reference patent? If yes, the subject matter is not identical and no statutory rejection applies.

A statutory double patenting rejection can only be cured by canceling or amending the conflicting claims so they are no longer coextensive in scope. A terminal disclaimer cannot cure this type of rejection.

Nonstatutory (obviousness-type) double patenting

Obviousness-type double patenting (ODP) is a judicially created doctrine designed to prevent unjustified extension of patent exclusivity beyond the term of a single patent. It applies when two sets of claims are not identical but one set is not patentably distinct from the other.

The doctrine serves two distinct policy goals: preventing unjustified timewise extension of the right to exclude, and preventing the possibility of multiple harassment suits by different assignees holding patents on patentably indistinct variations of the same invention.

The ODP analysis

Examiners evaluate ODP using the factual inquiries from Graham v. John Deere Co. that govern standard obviousness under 35 U.S.C. § 103: the scope and content of the reference claim versus the examined claim, the differences between them, the level of ordinary skill in the art, and any objective indicia of nonobviousness. The ODP analysis is similar to, but not necessarily the same as, a standard § 103 analysis.

Overcoming an ODP rejection: the terminal disclaimer

An applicant facing an ODP rejection can file a terminal disclaimer under 37 CFR § 1.321(c). To be effective, the terminal disclaimer must include a provision that the granted patent will be enforceable only for and during the period that it is commonly owned with the reference patent or application. The disclaimer is binding on the applicant's successors and assigns.

The practical consequence: if the two patents end up in separate hands after a sale or assignment, the later patent (the one subject to the disclaimer) loses enforceability. This directly prevents the multiple-assignee scenario that the ODP doctrine was designed to stop.

A terminal disclaimer can overcome a nonstatutory double patenting rejection. It cannot overcome a statutory double patenting rejection.

The § 121 safe harbor for divisional applications

When a restriction requirement forces an applicant to split claims into separate divisional applications, those divisionals receive special protection. Under 35 U.S.C. § 121, a patent issuing on an application subject to a restriction requirement may not be used as a reference against a divisional application, either at the USPTO or in courts, provided the divisional is filed before the other patent issues. This safe harbor is specific to divisional applications arising from a restriction requirement; the statute does not extend the same protection to continuation applications.

The practical benefit: sibling patents in a family split by a restriction requirement cannot be used against each other on double patenting grounds. If you are prosecuting a divisional that arose from a restriction, you have statutory protection from a double patenting rejection based on the parent or co-pending sibling applications.

Comparison at a glance

Practical notes for practitioners

Watch for ODP rejections when prosecuting continuation or continuation-in-part applications that pursue claims differing only modestly from an issued parent patent. Before filing a terminal disclaimer, weigh the permanent enforceability tie it creates against the option of amending claims to achieve patentable distinction. Amending avoids the common-ownership requirement but may narrow your claim scope. The terminal disclaimer route preserves broader claims but binds all future owners of both patents together for enforcement purposes. Neither choice is universally better; the right answer depends on your ownership structure and licensing strategy.