Check the descriptions of the programs. If they do not have a NEW or REPEAT tag, it will default to recording as if new. This is a long-time issue with the networks providing the correct data to the company (Gracenote I think) that provides the guide data for Comcast.
I am have had the same issue with our TiVo for the last several days. It is definitely a metadata problem but haven't had time to download a duplicate and cross check the metadata. I'll let you know what I find, but it is still the metadata source that has to fix the problem.