It looks like you can assign materials to curves. When doing so you get zillions of instances of the materials in your library.
Open the file.
Run Purge (materials won’t purge)
Delete the curves
Run Purge again (materials will purge)
What version of Rhino are you using? It looks like v7 as I see a PBR material in the screenshot but I tried to reproduce the issue shown in your file with the same groups of curves and only get one material swatch in the Material editor after assigning it. Do you have a series of steps that make this happen?
I tracked down the reproduceable issue:
In the attached file, the lines are grouped to the back surface of the fireplace.
Look for an image in Windows Explorer and drag it onto the group.
voila! The materials populate in the Material panel.
I’m using Version 7 SR10
LinesMaterialBug.3dm (16.7 MB)