I think I'm well and truly overwritten as regards duplication of PET cassettes, and whether it can be done by analogue means.
As Owen suggests, it is better to capture it in the form of a digital file which can be written back to a tape at some point, although this is an interesting point: Colin's tapes and the hardware to replay them are still working after 40+ years - can we honestly be sure that we will still have something which will play .WAV or .MP3 files in 40 years? For most of us here, this is fortunately not something we are likely to need to worry about.