Multiple members can have the same mail address

[dsluijk]

Reproductie

1. Create a member with a mail address.
2. Create a second member with the same mail address.
3. Observe that this is allowed by the server.

Beschrijf de bug

Multiple members can have the same mail address, which can cause issues down the line (e.g. authentication). It would be better if we can disallow two members to share the same mail address.

Extra context

No response

Logs

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