It could be to do with missing program guide data, some channels give very little information about programs and don't specify exactly which episode any showing is, this can lead to the same program being recorded multiple times as the box doesn't know they are the same episode.
indeed - as Tavis says, the TiVo uses EPG data to ID individual program episodes to avoid recording duplicates. If the EPG data is incomplete, then the default position is to record everything "in case".
Do you have any specific examples? Reason for asking, most of the mainstream channels are pretty good with their guide data, but mistakes do happen with minority channels especially.
There is an email inbox for reporting genuine guide errors, mainly for when the Virgin EPG differs from broadcasters' published ones etc.
Yes I think that we get the point that you are experiencing some sort of issue with recordings, but what exactly are the shows that you are experiencing issues with.
Some issues can be resolved with series links by setting recordings only, as this will preclude duplicate shows that are available via "On Demand" services. Some shows may have the same episode number but a slightly different name and as previously stated the V6 will record them, just in case they are different.
The program guide (as supplied by tv companies) can be inaccurate, which is totally outside Virgin's control.
There are many factors with series links, but without specifics of a show that you have an issue with it does tend to make assistance from the forum difficult.
You can't learn anything from a mistake until you admit that you've made it.