ID based contact or member
For Child based sports club, a contact / member account should be created per child, but linked to parent for paying and allowing membership options at a family level, as parent is not a member, just the legal payee. Also Key for membership account should be based on Identity number and not email, therefore allowing same email account on multiply contacts / members ( that is for the parent) - duplication of account is then on the ID no.
I agree 100% - folk sometimes share emails but also sometimes have to change them e.g. students use Uni email to start but when they leave Uni they lose their Uni email or someone has to change email due to too much spam. All kinds of reasons why email should not be a Primary Key. Really disappointed in this as I am thinking of moving to WA from a provider whose data is badly designed and who also uses email as a key. So easy to use a separate key.