The EPC does not deal explicitly with the case of co-pending European applications of the same effective date filed by the same applicant. However, it is an accepted principle in most patent systems that two patents cannot be granted to the same applicant for one invention. The Enlarged Board of Appeal has accepted obiter dictum that the principle of the prohibition on double patenting is based on the notion that an applicant has no legitimate interest in proceedings leading to the grant of a second patent for the same subject-matter if the applicant already possesses one granted patent for that subject-matter (see G 1/05, and G 1/06).

The prohibition of double patenting applies to three types of combinations of European applications by the same applicant: two applications filed on the same day, parent and divisional applications, or an application and its priority application.

It is permissible to allow an applicant to proceed with two applications having the same description which do not claim the same subject-matter (see also T 2461/10). The applicant may, for example, be interested in obtaining a first quicker protection for a preferred embodiment and pursue the general teaching in a divisional application (see G 2/10). However, in the rare case in which there are two or more European applications from the same applicant designating the same state or states and the claims of those applications have the same filing or priority date and relate to the same invention, the applicant should be told to perform one of the following: amend one or more of the applications in such a manner that the subject-matter of the claims of the applications is not identical, or withdraw overlapping designations, or choose which one of those applications is to proceed to grant. If the applicant does not do so, once one of the applications is granted, the other(s) will be refused under Art. 97(2) in conjunction with Art. 125. If the claims of those applications are merely partially overlapping, no objection should be raised (see T 877/06). Should two applications of the same effective date be received from two different applicants, each must be allowed to proceed as though the other did not exist.