Allow second participant registration with same email, when enabled

[larssandergreen]

Overview

When "Same email address" is enabled for an event, participants with the same email address as an existing participant should be allowed to register. They were, when they were the primary participant, but not when they were an additional participant.

Before

Attempting to register an additional participant with an email address for an existing participant results in an error, and the registration cannot not be completed without changing or skipping the participant.

After

No error, registration proceeds as normal.
If "Same email address" isn't enabled, no change.

[civibot]

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[demeritcowboy]

The intent of the original seems to be to prevent the same contact being registered twice, regardless of the email setting. It looks like it uses the dedupe rule you choose on the event config to determine this. While the existing restrictions seem a bit iffy it seems like this is a bigger change than just allowing a different participant with the same email?

[larssandergreen]

Preventing the same contact from being registered twice does seem to be the intent of the original. But if Same email address is checked, we allow a contact who is already registered to register again as the primary participant, so we should also allow this for the additional participants. This change is just making the logic consistent for primary and additional participants.

You could definitely argue that these should be two different settings (Same email address and Allow multiple registrations), but that's a bigger change and would open a whole other can of worms (what do you do if you have an email-only dedupe rule, don't allow same email, but do allow multiple registrations, etc). I also have a PR to clarify what this setting does.

[demeritcowboy]

Let me look at the history. Maybe there is something interesting there.

[demeritcowboy]

Ok:

• The dedupe rule was a bit of a red-herring because the participants may not even exist yet as contacts in which case it doesn't find them anyway.
• The only use-case I can see in the history is, paraphrasing: "The primary user often does something silly like registers themselves both as primary and additional participant, so we want to prevent that." There is an additional check lower down though that still catches this if they use the same first+last name.

[larssandergreen]

Thanks @demeritcowboy. Yes, I think the first + last check is a more appropriate method to prevent people from registering themselves twice on the same form.

[agileware-justin]

Agree with this PR.

Will need CIviCRM documentation updates as well, when merged.

[larssandergreen]

@agileware-justin I think the documentation as it stands now covers this.