Are T Tool icons accessible like the other icons? Why are there duplicate icons in the table?
As stated at developer.blender.org task toolbar icons are generated from a icon_geom.blend blend file and stored as actual 3D meshes. These are then rasterized into binary .dat icon files at compilation time and presented by the UI.
They are not part of the vector SVG icons created by jendrzych.
Duplicate icons are probably for use at different places in the UI. Either they were different before recent UI redesign and unified, or we might at some point in the future want to use different icons in different situations.