Often there is a repeating of episodes broadcasted at a later time.
If you use series schedules like "Everytime on this channel" you likely end up with duplicates of every single episode.

There are several ways to circumvent this:
1. The best way would obviously be to persist some history details about every recording and check this history. This approach however has to deal with lots of exceptions and special cases (e.g. it may be desired or not to record some schedule later again, EPG data might differ for repeatings, etc).
2. A more straightforward approach could simply check the existing recordings for the episode title (needs EPG data of course) of the upcoming schedule. If it is found and start time of schedule and recording differ (same time would indicate a crashed TV service and recording should be resumed) then the schedule will not get recorded again.