An inventor is only entitled to one patent for their invention. It is an improper extension of the patentee's monopoly to have more than one patent for the same invention, in particular because a patentee may otherwise be able to “evergreen” protection for a single invention by filing successive applications. Patent law therefore prohibits against “double patenting”. There are two “types” of double patenting, both of which are prohibited - “same invention” double patenting (two patents with identical or coterminus claims) and “obviousness-type” double patenting (where the subsequent patent claims are not “patentably distinct” or “obvious” from the earlier patent claims).

There is no explicit statutory prohibition against double patenting. However, s. 27(1) of the Patent Act provides that:

27 (1) The Commissioner shall grant a patent for an invention to the inventor or the inventor’s legal representative if an application for the patent in Canada is filed in accordance with this Act and all other requirements for the issuance of a patent under this Act are met. [Emphasis added.]¹

There are two “branches” of double patenting, “same invention” and “obviousness-type”, both of which are prohibited due to the concern that patentees may “evergreen” patent protection for their invention.² In both cases, the point of comparison between the first patent and the second patent are the claims, since double patenting is concerned with the improper extension of a patentee's monopoly.³ As Hughes J. stated in Merck v Pharmascience:⁴

What is important to keep in mind is that the exercise required in the inquiry as to whether there is double patenting is that the claims of the earlier patent owned by the same patentee as the latter must be compared with the claims of the latter to see if they are “identical or co-terminus”, or whether the latter is “obvious” in view of the former. Therefore, the exercise is somewhat different than that of dealing with obviousness of a patent having regard to the art that would have been known to a person skilled in the art as of the relevant time. The exercise respecting double patenting is to present the notional person skilled in the art with the claims of the first patent and inquire whether what is claimed in the second patent was “identical or co-terminus” with the first or would have been obvious in light of the earlier patent. The inquiry must not bother with any inquiry as to whether the earlier patent would have come to the attention of the notional person skilled in the art. Nor does the inquiry extend to the validity or otherwise of the claims of the earlier patent. Nor does the inquiry extend to “prior art” beyond the earlier patent, as Binnie J. wrote at paragraph 67 of Whirlpool, the inquiry is whether a second patent can be justified unless the claims exhibit “novelty or ingenuity” over the first patent. [Emphasis in original]

In comparing the claims, the ordinary principles of claims construction applies.⁵

If a subsequent patent issues with “identical or coterminous” claims as an earlier patent by the same patentee, the subsequent patent is invalid.⁶ This formulation in Canada is said to derive from a case comment of Dr. Harold G. Fox, where he stated:

Letters patent are not granted at pleasure, but for a term of years and the grant of a second patent with respect to the same subject-matter would be void under this statute [6 Henry VIII, c. 15, 1514] and by the Statute of Monopolies, as well as at common law and by the terms of section 28(1)(b) of the Canadian Patent Act. But for this purpose the subject-matter of the two grants must be identical. A subsequent claim cannot be invalidated on the ground of prior claiming unless the two claims are precisely conterminous.⁷

The test of whether the claims of the two patents are “identical and coterminous” is akin to the test for anticipation.⁸

The second branch of the prohibition is called “obviousness” double patenting. This is a more flexible and less literal test that prohibits the issuance of a second patent with claims that are not “patentably distinct” from those of the earlier patent.⁹ The term “not patentably distinct” is synonymous with other judicial formulations of the test, such as “improper extension”,¹⁰ “lack of ingenuity”,¹¹ or “no further invention”¹² over the previous patent. There is no substantive distinction between these approaches - the core analysis of obviousness-type double patenting is whether there is invention or ingenuity in the move from the first patent to the second.¹³

In other words, the legal test for obviousness-type double patenting follows the same analytical process as obviousness,¹⁴ as set out in Apotex Inc v Sanofi-Synthelabo Canada Inc:¹⁵

1. - Identify the notional “person skilled in the art”;

1. Identify the relevant common general knowledge of that person;
2. Identify the inventive concept of the claim in question or if that cannot readily be done, construe it;
3. Identify what, if any, differences exist between the matter cited as forming part of the “state of the art” and the inventive concept of the claim or the claim as construed;
4. Viewed without any knowledge of the alleged invention as claimed, do those differences constitute steps which would have been obvious to the person skilled in the art or do they require any degree of invention?

While the obviousness test applies, there are important distinctions to make for double patenting.

First, for double patenting, the only relevant “prior art” for comparison are the claims of the first patent, which are to be compared to the claims of the impugned second patent.¹⁶ Other prior art could be relevant insofar as it has become part of the “common general knowledge” of the skilled person, since the obviousness analysis is conducted from the perspective of a person of ordinary skill in the art.¹⁷

Second, double patenting is not subject to s. 28.3(a) of the Patent Act, meaning that there is no one-year “grace period” for the patentee's prior patent - thus the earlier patent can be cited for double patenting even if it was published within one year of the filing date of the second patent.¹⁸

The relevant date for determining double patenting is unsettled. There are at least three possible dates to consider: (1) priority date of the first patent; (2) priority date of the second patent; and (3) publication date of the second patent.¹⁹ The date chosen may have important ramifications for patentees. The policy justification for each date are different, and each have different merits and drawbacks.²⁰

In Mylan Pharmaceuticals ULC v Eli Lilly Canada Inc, the Federal Court of Appeal definitively rejected the publication date of the second patent. The Court held that any date after the claim date of the second patent is inappropriate, since that date would allow a court to consider prior art published after the claim date which it would not otherwise be able to do for the purpose of obviousness and thereby circumvent s. 28.3(a) of the Patent Act.²¹

The doctrine of double patenting can apply to invalidate divisional patents.²² Inventive ingenuity is thus required to support the “second” patent even where it is a divisional from the “first” patent.²³ The prohibition against double patenting for divisional patents indicates that the judicial concern is not solely based on “evergreening”, since divisional applications under modern Canadian patent law necessarily have the same expiry date as the original application.²⁴

Under the pre-1989 Patent Act, the expiry date of a patent was calculated from the date of issue, making evergreening a more significant concern for divisional patents. However, it is clear that improper extension of patent term was not the only reason to prohibit double patenting. Indeed, even under the “Old Act”, a terminal disclaimer limiting the term of the divisional patent to the expiry date of the original patent was insufficient to alleviate concerns for double patenting.²⁵

At least two additional reasons have been cited to prohibit double patenting in the case of divisional patents. One is that a divisional application without inventive ingenuity over the original patent would impermissibly extend the scope of the patent protection granted to the patentee.²⁶ A second reason is that patentees may gain other advantages by being able to assert more than one patent, such as the commencement of multiple prohibition proceedings under the Patented Medicines (Notice of Compliance) Regulations which could prolong the statutory injunction granted thereunder.²⁷

Where the divisional patents are a result of a requirement by the Patent Office,²⁸ it appears that any potential “double patenting” caused thereby should not invalidate the subsequent divisional patent by virtue of that reason alone since it would be prejudicial and unfair to hold the patentee responsible for an incorrect decision of a patent examiner.²⁹ This result appears to be a logical corollary of s. 36(1) of the Patent Act, which prevents a finding of invalidity solely on the grounds that a patent may claim more than one invention.³⁰