My 625 didn't do that the last time I checked. When we select a series, the guide data is supposed to have a bit set for "series", and the recording is automatically set to "all" rather than "once." Are you noticing whether it's actually setting itself to "once" for your series? If so, then it's a guide problem and recording another series (on another channel?) might work as expected.
The problem I've often had is the reverse of yours! When I try to record "all" of a series such as Spongebob Squarepants or Curious George, where the guide data is populated with generic info, rather than info specific to a channel, then I literally get recordings of every instance of the same program, often 2-8X on the same day. "All" is supposed to check what you have already recorded and avoid recording the same show over and over. Guide problem.