uv_vertex_count_diff.py

import bpy

obj = bpy.context.active_object

# Store all UV vertex locations
uv_coords = []
for loop in obj.data.loops:
    uv_loc = obj.data.uv_layers.active.data[loop.index].uv
    uv_coords.append(tuple(map(lambda x: round(x,3), uv_loc[:])))

# Keep only the unique ones and count them
print("object {0} has {1} UV vertices".format(obj.name, len(set(uv_coords))))

dupes = len(set(uv_coords)) - len(obj.data.vertices)
print("the number of duplicate verts is {0}:".format(dupes))