L2 after F1 - without cancellation/void stamping on F1

[srividya.swami]

Hi,

I have a F1 visa valid unti 21 July 2014. I had graduated in 2010 and returned to India.

I was issued L2 visa by Chennai Consulate on 6 September 2013. I heard that F1 visa should have been stamped cancelled/void, but there's no such stamp on the F1 visa on my passport.

Could someone confirm if that cancellation stamping is mandatory (by the US consulate), if so, please advice on steps that I should take.

Thank you!