Select all overlapping vertices

Question

There is "remove doubles" function for removing overlapping vertices and it works nice. But sometimes I'd like to see which vertices will be merged in order to decide whether I actually need to remove them or not. So in this case selecting them first (specifying distance threshold) to highlight would be useful. Unfortunately I have not found any such option in "select all by trait"

Answer 1

Ok, here the script that adds button with such function to "Select" > "Select All by Trait" menu. Feel free to improve.

Edit: Updated to blender 3.4.1

bl_info = {
    "name": "Select Doubles",
    "author": "Lambdadelta",
    "version": (1, 1),
    "blender": (3, 4, 1),
    "location": "(Edit Mode) Select > Select All by Trait",
    "description": "Select all overlapped vertices.",
    "warning": "",
    "tracker_url": "https://blender.stackexchange.com/questions/68326",
    "category": "Mesh",
}


import bpy, bmesh
from mathutils import kdtree


def select_by_distance(context, distance):
    ob = context.object
    me = ob.data
    bm = bmesh.from_edit_mesh(me)
    verts = bm.verts

    verts.ensure_lookup_table()

    size = len(verts)
    kd = kdtree.KDTree(size)
    for i, v in enumerate(verts):
        kd.insert(v.co, i)
    kd.balance()

    for v in verts:
        v.select = False

        vert_in_range_ids = [index for co, index, dist in kd.find_range(v.co, distance)]

        if len(vert_in_range_ids) > 1:
            for i in vert_in_range_ids:
                verts[i].select = True

    context.tool_settings.mesh_select_mode = True, False, False
    bm.select_flush_mode()
    bmesh.update_edit_mesh(me, loop_triangles=False, destructive=False)

class MESH_OT_select_by_distance(bpy.types.Operator):
    """Select duplicate vertices"""

    bl_idname = "mesh.select_doubles"
    bl_label = "Select by Distance"
    bl_options = {'REGISTER', 'UNDO'}

    distance: bpy.props.FloatProperty(
        name="Distance",
        subtype='DISTANCE',
        default=0.0001,
        min=0.0,
        max=50.0,
        description="Minimum distance between elements to select",
        unit='LENGTH',
    )

    @classmethod
    def poll(cls, context):
        ob = context.active_object
        return ob and context.mode == 'EDIT_MESH'

    def execute(self, context):
        select_by_distance(context, self.distance)
        return {'FINISHED'}

    def invoke(self, context, event):
        select_by_distance(context, self.distance)
        return {'FINISHED'}


def menu_func(self, _):
    self.layout.operator(MESH_OT_select_by_distance.bl_idname, text="Verts by Distance")


def register():
    bpy.utils.register_class(MESH_OT_select_by_distance)
    bpy.types.VIEW3D_MT_edit_mesh_select_by_trait.append(menu_func)


def unregister():
    bpy.utils.unregister_class(MESH_OT_select_by_distance)
    bpy.types.VIEW3D_MT_edit_mesh_select_by_trait.remove(menu_func)


if __name__ == "__main__":
    register()