ReadyNAS Duo v2 loads wrong firmware (4.1.16 SPARC instead of RAIDiator 5.x ARM)

Then your Duo is not really a V2 and cannot run OS5.x.  Netgear really messed up with the naming on the Duo and NV+ V2.  They had previously used a "v2" (normally lower case v) designation on a sticker to indicate a minor hardware change in the original Duo and NV+ lines (which we refer to here as a "V1", though that was never part of their official names).  Much later, they came out with the official "V2" lines, which were a completely different architecture.  This has confused buyers of used equipment ever since, as it has you.  Sellers who were not the original owners may also be confused.

The "real" V2 says "ReadyNAS V2" on the front.  Any other labeling is immaterial.  On a Duo, the original (aka V1) also has a recess for the power plug and a fan less than the full width of the unit where the V2 has no recess and a larger fan.