Go to the 3D View menu, Select > Select Pattern…, then type ‘*.001’, press Enter, then delete them all.
Select the Armature object, switch to Edit Mode, go to Outliner, set Type to Active, then search .001 (the naming suffix to each duplicate and automatically given by Blender). Then B to select all bones, then go to 3D View, press X, and choose Delete Bones.
First, select the root bone, then press Shift G, choose Children, then delete them all. (For those with no children, usually, you have to select them manually)