The very rare double overprint on cover is supplied with an RPS Victoria (Royal Philatelic Society) certificate.
Two sheets of the 3d was issued with double overprint. The second impression is weaker than the first, and is 1mm to the left, and slightly higher than the first overprint.
Examples of a similar, but a closer double overprint (from another sheet)have been used on cover cancelled at No. 8 Aust. Base P.O. -7 Nov 1947, shown below. This cover is one of only two existing.