Siemens S7 SIL2 query

Hello Siemens experts there,

A quick query regarding SIL2

Is it manadatory to setup redundant controllers to achieve SIL2 ? Or a standalone controller can be SIL2 complaint with Siemens PLCs?

I'm not that familliar with SIL but I'd think that if the risk assesment highlighted PLC failure as being dangerous then you ought to be considdering a failsafe PLC rather than redundancy.

Nick