don't fail silently if trying to add a collaborator twice

Author: adelowo

options/locale/locale_en-US.ini

@@ -1014,6 +1014,7 @@ settings.transfer_succeed = The repository has been transferred.
 settings.confirm_delete = Delete Repository
 settings.add_collaborator = Add Collaborator
 settings.add_collaborator_success = The collaborator has been added.
+settings.add_collaborator_duplicate = The collaborator has been previously added.
 settings.delete_collaborator = Remove
 settings.collaborator_deletion = Remove Collaborator
 settings.collaborator_deletion_desc = Removing a collaborator will revoke their access to this repository. Continue?

routers/repo/setting.go

@@ -401,6 +401,12 @@ func CollaborationPost(ctx *context.Context) {
 		}
 	}
 
+	if got, err := ctx.Repo.Repository.IsCollaborator(u.ID); err == nil && got {
+		ctx.Flash.Error(ctx.Tr("repo.settings.add_collaborator_duplicate"))
+		ctx.Redirect(ctx.Repo.RepoLink + "/settings/collaboration")
+		return
+	}
+
 	if err = ctx.Repo.Repository.AddCollaborator(u); err != nil {
 		ctx.ServerError("AddCollaborator", err)
 		return