Weird play - curious about rule

[RKBUmp]

With no IFF rule in effect, the runner on 3rd lost the right to the base on the fair batted ball. Even standing on it she no longer has the right to the base and can be tagged out. With the tag and then step on the base, it was a double play. If the fielder had stepped on the base first, the force would have been removed and the runner standing on 3rd would have been safe on the tag. If both players were standing on the base, the runner originally on 3rd lost the right to the base on the hit and could be tagged out, stepping on the base would do nothing as the advancing runner is already there and safe.