Having multiple contours in the same slice in the same structure does not seem to be valid. DICOM standard Part 3 C. 8.8.6 does not explicitly say that intersection is prohibited, but “closed contour (polygon) containing coplanar points” indicates that a simple polygon is expected. It is explicitly stated that you cannot even make a hole in a polygon (you need to use keyhole technique instead).
Did you draw multiple contours in the same slice in the same structure in the treatment planning system? Or you have distinct contours in the treatment planning system but they show up at the same slice in Slicer?