diff --git a/app/controllers/form_controller.rb b/app/controllers/form_controller.rb
index 965464f35..125671d02 100644
--- a/app/controllers/form_controller.rb
+++ b/app/controllers/form_controller.rb
@@ -175,7 +175,7 @@ private
       if dynamic_duplicates.count.positive?
         saved_duplicates = @log.duplicates
         unless saved_duplicates == dynamic_duplicates
-          @log.update!(duplicate_set_id: new_duplicate_set_id) if @log.duplicate_set_id.blank?
+          @log.update!(duplicate_set_id: new_duplicate_set_id(@log)) if @log.duplicate_set_id.blank?
           dynamic_duplicates.each do |duplicate|
             duplicate.update!(duplicate_set_id: @log.duplicate_set_id) if duplicate.duplicate_set_id != @log.duplicate_set_id
           end
@@ -293,10 +293,11 @@ private
     SalesLog.find_by(duplicate_set_id:)&.update!(duplicate_set_id: nil) if log.sales? && SalesLog.where(duplicate_set_id:).count == 1
   end
 
-  def new_duplicate_set_id
-    loop do
-      duplicate_set_id = SecureRandom.random_number(1_000_000)
-      return duplicate_set_id unless LettingsLog.exists?(duplicate_set_id:)
+  def new_duplicate_set_id(log)
+    if log.lettings?
+      LettingsLog.maximum(:duplicate_set_id).to_i + 1
+    else
+      SalesLog.maximum(:duplicate_set_id).to_i + 1
     end
   end
 end
diff --git a/lib/tasks/set_duplicate_references.rake b/lib/tasks/set_duplicate_references.rake
index d1fc9b18c..82f75f031 100644
--- a/lib/tasks/set_duplicate_references.rake
+++ b/lib/tasks/set_duplicate_references.rake
@@ -1,7 +1,7 @@
 desc "Set duplicate references for sales and lettings logs"
 task set_duplicate_references: :environment do
   SalesLog.filter_by_year(2023).duplicate_sets.each do |duplicate_set|
-    duplicate_set_id = generate_new_duplicate_set_id
+    duplicate_set_id = SalesLog.maximum(:duplicate_set_id).to_i + 1
     next if duplicate_set.any? { |_log_id| SalesLog.exists?(duplicate_set_id:) }
 
     duplicate_set.each do |log_id|
@@ -12,7 +12,7 @@ task set_duplicate_references: :environment do
   end
 
   LettingsLog.filter_by_year(2023).duplicate_sets.each do |duplicate_set|
-    duplicate_set_id = generate_new_duplicate_set_id
+    duplicate_set_id = LettingsLog.maximum(:duplicate_set_id).to_i + 1
     next if duplicate_set.any? { |_log_id| LettingsLog.exists?(duplicate_set_id:) }
 
     duplicate_set.each do |log_id|
@@ -22,10 +22,3 @@ task set_duplicate_references: :environment do
     end
   end
 end
-
-def generate_new_duplicate_set_id
-  loop do
-    duplicate_set_id = SecureRandom.random_number(1_000_000)
-    return duplicate_set_id unless LettingsLog.exists?(duplicate_set_id:)
-  end
-end