fix: prevent duplicate waiting list entries

.gitignore

@@ -58,3 +58,6 @@ vendor/bundle
 /yarn-error.log
 yarn-debug.log*
 .yarn-integrity
+
+# Git worktrees
+.worktrees

app/models/waiting_list.rb

@@ -9,7 +9,9 @@ class WaitingList < ApplicationRecord
   scope :with_notes_and_their_authors, -> { includes(member: { member_notes: :author }) }
 
   def self.add(invitation, auto_rsvp = true)
-    create(invitation: invitation, auto_rsvp: auto_rsvp)
+    find_or_create_by(invitation: invitation) do |waiting_list|
+      waiting_list.auto_rsvp = auto_rsvp
+    end
   end
 
   def self.students(workshop)

db/migrate/20260211140256_add_unique_index_to_waiting_lists_invitation_id.rb

@@ -0,0 +1,54 @@
+class AddUniqueIndexToWaitingListsInvitationId < ActiveRecord::Migration[8.1]
+  def up
+    # Clean up duplicate waiting list entries
+    duplicate_count = say_with_time "Cleaning up duplicate waiting list entries" do
+      duplicate_invitation_ids = WaitingList
+        .group(:invitation_id)
+        .having('COUNT(*) > 1')
+        .pluck(:invitation_id)
+
+      duplicate_count = duplicate_invitation_ids.count
+      say "Found #{duplicate_count} invitation_ids with duplicates"
+
+      if duplicate_count > 0
+        # For each duplicate set, keep oldest and delete the rest
+        duplicate_invitation_ids.each do |invitation_id|
+          entries = WaitingList
+            .where(invitation_id: invitation_id)
+            .order(:created_at)
+
+          # Get IDs to delete (all except first/oldest)
+          ids_to_delete = entries[1..].map(&:id)
+          deleted_count = ids_to_delete.size
+
+          say "  Invitation #{invitation_id}: deleting #{deleted_count} duplicate(s), keeping entry ##{entries.first.id}"
+
+          # Use delete_all for performance and to avoid callbacks
+          WaitingList.where(id: ids_to_delete).delete_all
+        end
+      end
+
+      duplicate_count
+    end
+
+    # Add unique constraint (remove existing non-unique index first if it exists)
+    say_with_time "Adding unique index on waiting_lists.invitation_id" do
+      begin
+        remove_index :waiting_lists, :invitation_id
+      rescue StandardError => e
+        say "  Note: Could not remove existing index (#{e.message})"
+      end
+
+      add_index :waiting_lists, :invitation_id, unique: true
+    end
+  end
+
+  def down
+    begin
+      remove_index :waiting_lists, :invitation_id
+    rescue StandardError => e
+      say "Could not remove index: #{e.message}"
+    end
+    # Note: Cannot restore deleted duplicate entries
+  end
+end