This does seem like a mystery.
Typically there would be no problem recording DV onto a tape that had already been used. Despite what the tape manufacturers want you to think; you can reuse DV tapes over and over until the magnetic coating physically wears down. (You can expect the tape to be good for at least 20 recordings.)
Given how these devices work, I would venture a guess that you were not actually recording to the tape when you thought you were. This is a situation that happens to the best of us.
Most modern camcorders are so quiet and efficient in the tape transport mechanism that you would barely hear if the tape is rolling- and it's easy to think your recording even if your not.
The other possible explanation for your problem is that you cued the tape up to a different portion of the tape. Maybe, your tape was not rewound all the way to the beginning when you started recording. Then, when you went to watch on your DV deck, you DID rewind all the way, and did not see what you expected.
If you find that you had a different problem, please let the forum know.
A few days ago, I recorded a video on an old DV tape we had lying around. Today, I recorded video over what was already on it. Upon review, I found that the tape deck only showed the old video and the camera doesn't show any video at all. Any ideas on what the problem could be? Thanks in advance.