sin(2x) - sin(x) = 0
Expand the first term using the double angle identity:
2 sin(x) cos(x) - sin(x) = 0
Factor out sin(x) :
sin(x) (2 cos(x) - 1) = 0
This leaves you with 2 cases that can be solved separately:
sin(x) = 0 or 2 cos(x) - 1 = 0
sin(x) = 0 or cos(x) = 1/2
[x = 2nπ or x = π + 2nπ] or [x = π/6 + 2nπ or x = 5π/6 + 2nπ]
(where n is any integer)