I have a colleague flying multiple segments on different days that are all on the same itinerary - LHR-ATL, ATL-STL, STL-MSP-DEN, DEN-ATL-LHR - next month over the course of a few weeks.
He did sign up for a Skymiles account, but we were talking about stuff and the issue of bag fees came up, so I called the DM line and asked about what he would have to pay.
As expected, the agent said 1 bag would be free on the LHR-ATL/DEN-ATL-LHR legs..as is normal. He will have to pay the $25 fee for ATL-STL and another fee for the STL-DEN, as they are on different days.
I just figured I'd point this out, as I'm sure there are folks out there looking to do similar sorts of trips over the holidays and people are not always aware of how baggage fees apply.