arun, that's called nesting. It used to happen a lot in Oz back in the days of "APEX" fares; before the current era of cheaper one way fares.
Consider the following: Say you need to visit SIN three times. Buy a PER-SIN-PER round trip for your first and last sectors. Call this T1 and the sectors T1S1 & T1S2. Then buy two SIN-PER-SIN round trips; call these T2 & T3.
Then travel:
Code:
T1S1 PER-SIN
T2S1 SIN-PER
T2S2 PER-SIN
T3S1 SIN-PER
T3S2 PER-SIN
T1S2 SIN-PER
Just be REALLY careful with your dates of travel.