If you attach a ideal string to a wall and pull it with force F, there should be a tension T=F in the string. And the string will apply force F on the wall too.
But what if there are two persons pulling the same string in opposite directions with same force F? Shouldn't the tension in the string be 2F since both persons are applying force in opposite directions. Or the another person which is pulling the string is acting as wall just like in first case?