diff --git a/db/migrate/20250522201031_stronger_unique_index_on_security.rb b/db/migrate/20250522201031_stronger_unique_index_on_security.rb
new file mode 100644
index 00000000..43672f81
--- /dev/null
+++ b/db/migrate/20250522201031_stronger_unique_index_on_security.rb
@@ -0,0 +1,13 @@
+class StrongerUniqueIndexOnSecurity < ActiveRecord::Migration[7.2]
+  def change
+    remove_index :securities, [ :ticker, :exchange_operating_mic ], unique: true
+
+    # Matches our ActiveRecord validation:
+    # - uppercase ticker
+    # - either exchange_operating_mic or empty string (unique index doesn't work with NULL values)
+    add_index :securities,
+              "UPPER(ticker), COALESCE(UPPER(exchange_operating_mic), '')",
+              unique: true,
+              name: "index_securities_on_ticker_and_exchange_operating_mic_unique"
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index dc54c285..c4d0be5f 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.2].define(version: 2025_05_21_112347) do
+ActiveRecord::Schema[7.2].define(version: 2025_05_22_201031) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pgcrypto"
   enable_extension "plpgsql"
@@ -523,9 +523,9 @@ ActiveRecord::Schema[7.2].define(version: 2025_05_21_112347) do
     t.datetime "failed_fetch_at"
     t.integer "failed_fetch_count", default: 0, null: false
     t.datetime "last_health_check_at"
+    t.index "upper((ticker)::text), COALESCE(upper((exchange_operating_mic)::text), ''::text)", name: "index_securities_on_ticker_and_exchange_operating_mic_unique", unique: true
     t.index ["country_code"], name: "index_securities_on_country_code"
     t.index ["exchange_operating_mic"], name: "index_securities_on_exchange_operating_mic"
-    t.index ["ticker", "exchange_operating_mic"], name: "index_securities_on_ticker_and_exchange_operating_mic", unique: true
   end
 
   create_table "security_prices", id: :uuid, default: -> { "gen_random_uuid()" }, force: :cascade do |t|
diff --git a/lib/tasks/data_migration.rake b/lib/tasks/data_migration.rake
index c84dbab0..509e033c 100644
--- a/lib/tasks/data_migration.rake
+++ b/lib/tasks/data_migration.rake
@@ -20,4 +20,95 @@ namespace :data_migration do
       puts "Error updating webhook for Plaid item #{item.plaid_id}: #{error.message}"
     end
   end
+
+  desc "Migrate duplicate securities"
+  # 2025-05-22: older data allowed multiple rows with the same
+  # ticker / exchange_operating_mic (case-insensitive, NULLs collapsed).
+  # This task:
+  #   1. Finds each duplicate group
+  #   2. Chooses the earliest-created row as the keeper
+  #   3. Re-points holdings and trades to the keeper
+  #   4. Destroys the duplicate (which also removes its prices)
+  task migrate_duplicate_securities: :environment do
+    puts "==> Scanning for duplicate securities…"
+
+    duplicate_sets = Security
+      .select("UPPER(ticker) AS up_ticker,
+               COALESCE(UPPER(exchange_operating_mic), '') AS up_mic,
+               COUNT(*) AS dup_count")
+      .group("up_ticker, up_mic")
+      .having("COUNT(*) > 1")
+      .to_a
+
+    puts "Found #{duplicate_sets.size} duplicate groups."
+
+    duplicate_sets.each_with_index do |set, idx|
+      # Fetch duplicates ordered by creation; the first row becomes keeper
+      duplicates_scope = Security
+                           .where("UPPER(ticker) = ? AND COALESCE(UPPER(exchange_operating_mic), '') = ?",
+                                  set.up_ticker, set.up_mic)
+                           .order(:created_at)
+
+      keeper = duplicates_scope.first
+      next unless keeper
+
+      duplicates = duplicates_scope.offset(1)
+
+      dup_ids    = duplicates.ids
+
+      # Skip if nothing to merge (defensive; shouldn't occur)
+      next if dup_ids.empty?
+
+      begin
+        ActiveRecord::Base.transaction do
+          # --------------------------------------------------------------
+          # HOLDINGS
+          # --------------------------------------------------------------
+          holdings_moved   = 0
+          holdings_deleted = 0
+
+          dup_ids.each do |dup_id|
+            Holding.where(security_id: dup_id).find_each(batch_size: 1_000) do |holding|
+              # Will this holding collide with an existing keeper row?
+              conflict_exists = Holding.where(
+                account_id: holding.account_id,
+                security_id: keeper.id,
+                date:        holding.date,
+                currency:    holding.currency
+              ).exists?
+
+              if conflict_exists
+                holding.destroy!
+                holdings_deleted += 1
+              else
+                holding.update!(security_id: keeper.id)
+                holdings_moved += 1
+              end
+            end
+          end
+
+          # --------------------------------------------------------------
+          # TRADES — no uniqueness constraints -> bulk update is fine
+          # --------------------------------------------------------------
+          trades_moved = Trade.where(security_id: dup_ids).update_all(security_id: keeper.id)
+
+          # Ensure no rows remain pointing at duplicates before deletion
+          raise "Leftover holdings detected" if Holding.where(security_id: dup_ids).exists?
+          raise "Leftover trades detected"   if Trade.where(security_id: dup_ids).exists?
+
+          duplicates.each(&:destroy!)   # destroys its security_prices via dependent: :destroy
+
+          # Log inside the transaction so counters are in-scope
+          total_holdings = holdings_moved + holdings_deleted
+          puts "[#{idx + 1}/#{duplicate_sets.size}] Merged #{dup_ids.join(', ')} → #{keeper.id} " \
+               "(#{total_holdings} holdings → #{holdings_moved} moved, #{holdings_deleted} removed, " \
+               "#{trades_moved} trades)"
+        end
+      rescue => e
+        puts "ERROR migrating #{dup_ids.join(', ')}: #{e.message}"
+      end
+    end
+
+    puts "✅  Duplicate security migration complete."
+  end
 end