WIP Costing for Lot based BOM

We have a situation where the cost for Parent Item WIP job is rolling up child Subassembly item's resource cost again,even though child item Unit cost has considered it.

The BOM and routing looks like this:

FG -> Item A

BOM-> Item B -Basis Item Qty 3.5

            Item B  Basis Lot Qty 17.5

Subassembly -Item B has components whose cost adds upto 1 and an OSP routing with resource cost of 1.

FG Item A doesnt not have any OSP operation

When cost rollup is done item B gets a cost of 2 (including the OSP cost) and Item A is getting Material Cost=2*21= 42 and a OSP resource cost=1.