renameCollection target should never have identical UUID with a different database

    // Check if the target namespace exists and if dropTarget is true.

    // Return a non-OK status if target exists and dropTarget is not true or if the collection

    // is sharded.

    Collection* targetColl = targetDB->getCollection(opCtx, target);

    if (targetColl) {

        // If we already have the collection with the target UUID, we found our future selves,

        // so nothing left to do but drop the source collection in case of cross-db renames.

        if (targetUUID && targetUUID == targetColl->uuid()) {

            if (source.db() == target.db())

                return Status::OK();

            BSONObjBuilder unusedResult;

            return dropCollection(opCtx,

                                  source,

                                  unusedResult,

                                  {},

                                  DropCollectionSystemCollectionMode::kAllowSystemCollectionDrops);

        }