I can invite a user to a project multiple times by email.

[P-Shakibafar]

When I am on the page of each project, I invite a user to the project by entering the user email and again and again I can do that and that shouldn't be the case.

[PresKhaled]

In 'ProjectInvitationController@store' we can use the relationship $project->members, like this:

if ($project->members->contains($user)) {
    // Add errors to invitation bag
}

but I can't add an error to the invitation errors bag, so I didn't write a test for it neither gets it to work