Double patenting is the granting of two patents for a single invention, to the same proprietor and in the same country or countries. According to the European Patent Office, it is an accepted principle in most patent systems that two patents cannot be granted to the same applicant for one invention. However, the threshold for double patenting varies from jurisdiction to jurisdiction.

Australian patent law includes a statutory bar on double patenting to the same inventor, but not to different inventors, for the same invention. Subsection 64(1) firstly grants the Commissioner of Patents a discretion to grant multiple patents for the same invention:

However, Subsection 64(2) prohibits such grant where multiple patents would be granted to the same inventor:

The statutory provisions contained in the Patents Act 1990 are a codification of the law established in the Dreyfus case. An application may be examined and accepted, but the double patenting provision will prevent a patent being granted on a second application unless and until the objection is removed by amendment of the offending claim or claims or surrender of the granted patent in favor of the second application.

In Canada, the Supreme Court has indicated that there are two branches of double patenting. The first branch of double patenting ("same invention" or "novelty-type" double patenting) requires that claims of two issued patents must not be identical or conterminous. The second branch of double patenting (termed "obviousness-type" double patenting) requires that claims of two issued patents be "patentably distinct" (non-obvious) in view of one another. Even applications that expire on the same day may be open to double patenting challenges, as the courts have viewed possession of two patents for one invention to constitute an undue benefit.

Technically, double patenting can only exist between issued patents. However, double patenting objections may be raised by an examiner during an application's pendency. The position of the Patent Office is that such objections are raised as a courtesy; examiners are under no obligation to identify double patenting situations during prosecution. Canadian practice differs significantly from U.S. practice on this front, in that, once a double patenting objection is encountered, there is no statutory provision for terminal disclaimer in the Canadian Patent Act. Thus, terminal disclaimer is not available to overcome double patenting objections. Instead, the applicant must demonstrate (by arguing or amending the claims) that the claims are patentably distinct in view of those of the co-pending application.

Divisional applications filed voluntarily may be susceptible to attacks on the grounds of double patenting. However, where a divisional is forced by the Patent Office during prosecution (i.e. to overcome a unity of invention objection), the Supreme Court of Canada has indicated that such an application and its parent should not be open to double patenting challenges.

The European Patent Convention (EPC) does not contain any specific provisions relating to double patenting. The European Patent Office's (EPO) Boards of Appeal addressed the issue in several cases however, and a referral to the Enlarged Board of Appeal was eventually announced in February 2019.