The DICOMITKSeriesGDCMReader has a list of DICOM Tags that are used to distinguish whether two DICOM images can be attributed to the same mitk::Image or not.
However, this code needs the tags to be absolutely identical as no numerical comparison is performed (only string based). In my case, this is a problem because one image has a slice-thickness of 4 and the neighbouring slice has a slice-thickness of 4.00000. In the current implementation, these DICOM images do not end up in the same mitk::Image.
The fix is quite simple, as a similar fix has already been implemented for the Tag 'Image Orientation'.