This java code:
        object.field = other_object;
        object.field = other_object;
should be optimized by c2 as:
        object.field = other_object;
but it's not with G1 enabled because GraphKit::g1_write_barrier_post() has:
// Use Op_MemBarVolatile to achieve the effect of a StoreLoad barrier.
insert_mem_bar(Op_MemBarVolatile, oop_store);

which confuses StoreNode::Ideal()

With the membar above commented out, the redundant store is optimized out but it seems we leave the pre/post barrier for the second store in.

The membar seems too coarse grain as all is required is that store load between the write to a field and the load from the card table.