A method includes forming a first mandrel pattern and a second mandrel pattern. The first mandrel pattern includes at least first and second mandrels for a mandrel-spacer double patterning process. The second mandrel pattern includes at least a third mandrel inserted between the first and second mandrels. The first mandrel pattern and the second mandrel pattern include a same material. The first and second mandrels are merged together with the third mandrel to form a single pattern.