I can subscribe again with my test account, in the My Subscriptions tab it shows only once, in Subscription History it shows I have subscribed twice.
I found out why it's possible, it's because the subscription is set to "Pending".
The reason I don't want to automatically activate the subscription is that the "payment" for the "free" plan is that they are active affiliates and have referred a required number of new users. So until they have referred the required number of new users they should not be able to access restricted content.
I was expecting that a pending subscriber would get a message like "You have already subscribed to this plan, the plan is pending activation"
Now when they click on restricted content they get the signup button again and sign up several times...