User info overwritten by new member

I created a contribution page to allow new members to sign up. Then I just signed up with the email of an existing member but other info (like a new name, address). As a result: the info of the existing member gets overwritten (name, address) in civicrm.
I just found out that the profile-setting for duplicate contacts is being ignored for online contribution pages (I don't really understand why ...), but even creating a new Duplicate Contact rule with type Unsupervised (specifying lastname,firstname and email) results in the same behavior.
So: is there any way of blocking new members from changing the info for already existing members via a regular contribution-membership page? Curretly I see this as a bug, please correct me if I'm wrong ...