This is a patch with the improvement suggested by csleight. He created the patch in response to his own request, but was unable to upload, so I'm attaching it for him.

Code changed in hudson
User: : krwalker
Path:
trunk/hudson/plugins/email-ext/src/main/java/hudson/plugins/emailext/EmailRecepientUtils.java
trunk/hudson/plugins/email-ext/src/main/java/hudson/plugins/emailext/ExtendedEmailPublisher.java
trunk/hudson/plugins/email-ext/src/test/java/hudson/plugins/emailext/EmailRecepientUtilsTest.java
http://jenkins-ci.org/commit/37048
Log:
JENKINS-8071 Use a Set instead of a List so email addresses are unique.

It would be great to see this patch rolled into the next release of the extremely useful email-ext extension.

This patch wasn't committed and this useful feature is still absent.

Also if we have several triggers with the same user, there should be only 1 e-mail sent to each user.

Yes, it was committed. This issue was for a single trigger. If the user shows up in multiple triggers, they will get multiple emails. That will not change.

This patch wasn't committed, see master branch on GitHub - changes are absent.

A simpler change was made to use a Set instead of a List. It still only affects a single trigger.

I see now. Why there will be no change to make e-mails unique across all triggers?

The whole point of multiple triggers is to send multiple emails.

It was already discussed somewhere? I can't find it. But as i see it - the whole point of multiple triggers is to trigger for multiple events and it doesn't refer to e-mails count at all.

But in most cases triggers produce exactly the same e-mails (however it is just default) and there is no reason to send several same e-mails for the same person. With current UI it's not easy to make e-mails unique across triggers.

The triggers each allow you to specify a list of people to send to, a different subject, a different body, even different attachment settings, so how is it hard to create a unique email per trigger?

Yes, that's my point - all trigger e-mails can be customized, so in current UI there is no clear way to resolve the problem. The problem is - there are many triggers and different subscribers, but some subscribers specified in several triggers and all triggers use base template (empty body, subject etc.), so if several triggers run in same time, user will receive several duplicate e-mails. And in current UI there is no way to prevent it.

The good way to fix this is - if trigger doesn't override any field (so it generates completely same e-mail as another trigger), then there should be only 1 such e-mail for specific subscriber. In more general case - we need to collapse generated e-mails by hash(body/subject/etc.) and then make recipient list unique by passing it in set.

That might meet YOUR specific need, but the general usage of the plugin would suffer from the change. Each trigger is treated completely separately on purpose.