Only validate for duplicate member email address when configured to do so

[AndyButland]

Prerequisites

• I have added steps to test this contribution in the description below

Fixes: #18745

Description

This feature was introduced in 13.7 via #16202 but it looks like it wasn't fully merged up to 15. I've added the missing piece in this PR such that we only validate for duplicate member email address when configured to do so.

To Test:

• Verify that you can or cannot create or update members with duplicate email addresses with the following configuration value set to false and true respectively.

  "Security": {
    "MemberRequireUniqueEmail": true|false,
    "UsernameIsEmail": false
  },

[nikolajlauridsen]

The fix itself seems to work as intended, however it uncovers a different bug, if I set both MemberRequireUniqueEmail to false, and UsernameIsEmail to false then I do get to create two members with the same mail, but different login names, however this then updates the name of the member for both of them as you can see in this video:

bug.mp4

Looking at the database I can also see that the member table looks correct, however the node text are updated incorrectly:

I appreciate that this is technically a seperate issue, but might be worth tackling first 😄

[AndyButland]

Good catch. Seems the problem came from a look-up by email that we do in creating members, and with this configuration that's not safe to do. I've switched it to do a look-up by username instead, as that will always be unique.

[nikolajlauridsen]

That did the trick, let's get this merged 👍