Double Patenting: One patent to rule them all

When you file a patent, you are asking for something powerful: an exclusive right to your invention for up to twenty years. That exclusivity is the reward for sharing your idea with the public. Yet, the patent system has built-in limits to keep things fair. One of the key limits is that a single invention should not give rise to multiple patents that unduly extend protection. This is known as the prohibition on double patenting.

Both the U.S. Patent and Trademark Office (USPTO) and the European Patent Office (EPO) apply this principle, though in different ways. The USPTO explicitly prohibits double patenting, while the EPO adopts the national laws of its member states to enforce the same outcome. The objective, however, is shared: to prevent patent owners from stretching their monopoly beyond what is reasonable.

What does double patenting mean?

At its simplest, double patenting means you can’t get two patents for the same invention. It’s like trying to lock the same door twice with the same key and keyhole: once it’s locked, it’s already protected. Locking it twice doesn’t double your protection in that case.

Two patents for one invention would burden patent offices with redundant examination, create uncertainty for competitors, and allow applicants to manipulate enforcement timelines. That’s why both the U.S. Patent and Trademark Office (USPTO) and the European Patent Office (EPO) enforce this rule, even if their methods differ.

How the USPTO handles double patenting

The U.S. patent system recognizes two types of double patenting:

Same-invention double patenting: when two applications claim exactly the same subject matter.
Obviousness-type double patenting: when the second application is not patentably distinct from the first (for example, a small or predictable modification).

The first type cannot be fixed; identical inventions simply cannot receive two patents.

The second type, however, can be resolved through a terminal disclaimer, a legal declaration stating that the later patent will expire at the same time as the earlier one and will remain under the same ownership.

This mechanism prevents applicants from extending their monopoly period, while allowing legitimate variations or improvements to coexist within the same ownership structure.

How the EPO addresses double patenting

The European Patent Office takes a stricter and more straightforward approach. It examines whether the claims of a later application cover the same subject matter as those of an earlier one. If they do, the claims of the later application are outright rejected.

Unlike in the United States, there is no “terminal disclaimer” procedure in Europe. The EPO’s view is clear: once an invention has been claimed and granted, that subject matter cannot be claimed again, even in a related application.

This ensures that patent protection across Europe remains clear, predictable, and finite.

Why the rule matters

The patent system is built on balance. In exchange for a time-limited monopoly, the inventor discloses the invention to society, allowing others to build upon it once the patent expires.

If applicants were allowed to file serial divisionals, continuations, or minor variations indefinitely, they could effectively “evergreen” their protection, prolonging exclusivity or creating uncertainty about when rights truly end. Double patenting rules are designed to stop exactly that. They ensure that once a patent’s term has run its course, competitors and the public can confidently use the invention.

In the U.S., this balance is maintained through the option of terminal disclaimers. In Europe, it’s achieved through a stricter one-patent-per-invention rule. Either way, the principle is the same.

In summary

Double patenting is not a procedural technicality, it’s a cornerstone of the patent system’s fairness.

Novelty and inventive step ensure you cannot patent what the world already knows.
The prohibition on double patenting ensures you cannot extend your own rights beyond their rightful term.

Whether under the flexibility of the USPTO or the rigidity of the EPO, the underlying message is universal: one invention, one patent, one term.