In principle, it is possible for the European Patent Office (EPO) to grant multiple patents on exactly the same invention, if the applications for those patents have the same date.
However, this is considered undesirable if the patents are from the same applicant. This is referred to as double patenting.
The European Patent Convention (EPC) does not explicitly prohibit such double patenting by the same applicant. On the contrary, a Board of Appeal found it possibly justified that the applicant can thus optimize his protection period: 20 years from the filing date of the first patent application, in addition to 20 years from the date of a second patent application filed one year later, but with the same priority.
However, in G4/19, the EPO Enlarged Board of Appeal ruled that such double patenting is prohibited without exception. Documents preparatory to the EPC showed that the delegations of the contracting states intended this to be the case, and the EPO is bound by it. After a patent is granted first, another patent application from the same applicant on the same subject matter must be rejected.
Best course of action
Therefore, if an applicant has two European patent applications of which the last one filed invokes the priority of the first one, the applicant is better off withdrawing the first European patent application before grant, or claiming different subject matter in the patent applications. This was true even before G4/13. The condition of the same subject matter set by G4/19 for the prohibition of double patenting does not apply to different subject matters between which there is partial overlap. A small difference is usually sufficient to avoid the prohibition.
For Dutch patents this is regulated differently. There, the patent on the first patent application loses its legal effect insofar as it relates to the same invention as the patent on the later application invoking priority. In contrast to Europe, where the last patent to be granted loses its validity, which usually concerns the last application, in the Netherlands the patent on the first application loses its validity.