I agree that we all gain by borrowing and sharing ideas. 'And sharing' is the operative word. We're all individual as artists--and as teachers, as well. I'm in a district where we share ideas and no two of us teach the same lesson the same.
THAT SAID, poaching someone's lesson idea and teaching it to a lower level class when you know those students will go on to be in the upper level class is a REALLY ROTTEN thing to do. In my old elementary school, the English teachers all had an agreement that books used for curriculum in one grade were not to be assigned at other grade levels.
If you're in this situation, I think your chair has to put curriculum mapping on the agenda of the next departmental meeting.