Double patenting refers to the granting of patent protection twice for the same invention. Generally, double patenting is not permitted because an inventor could file a later application for the same invention and receive another patent term of life.
In the U.S., double patenting is rejected under two circumstances:
1. when the same invention is requested to be patented again; and
2. in continuing patent applications.
If in a continuing patent application, a claim sought for in a second patent is an obvious variation only and nothing new, a patent will not be granted. When claims of variations of an invention are patented under two separate patents, it is seen that expiration of the first patent results in an improper extension of patent rights due to an unexpired second patent. The problem associated with double patenting can be overcome by a patentee issuing a “terminal disclaimer”. A terminal disclaimer is a written statement issued by the owner or patentee stating that s/he has disclaimed the period of second issued patent that would extend beyond the expiration of the first patent.
Double patenting also applies when there is a conflict of patent applications by two persons with regard to the same invention. When different parties claim identical invention, the Patent and Trademark Office (PTO) will require proof of first completion and the patent will granted to the party who completed the invention first.