Duplicate email when adding a contributor

[remram44]

Describe the bug

When I am added as contributor to a project, I get two emails:

• The one telling me I have been added as contributor (good)
• The one telling existing contributors that a new one has been added (bug)

Expected behavior

The second email should only be sent to previously existing contributors, otherwise a duplicate is always received by the new contributor (first impressions matter, right now the first thing you see when becoming a contributor is a bug)

To Reproduce

Add a new contributor to a project.

Additional context

cc @Mariatta #3155
#1000

[di]

Thanks @remram44!