diff --git a/spec/requests/form_controller_spec.rb b/spec/requests/form_controller_spec.rb
index ccc849d5e..80157e992 100644
--- a/spec/requests/form_controller_spec.rb
+++ b/spec/requests/form_controller_spec.rb
@@ -939,19 +939,141 @@ RSpec.describe FormController, type: :request do
               },
             }
           end
+          let(:page_id) { "buyer_1_age" }
 
-          context "and duplicate logs" do
-            let!(:duplicate_logs) { create_list(:sales_log, 2) }
+          context "when the answer makes the log a duplicate" do
+            context "with one other log" do
+              let(:new_duplicate) { create(:sales_log) }
 
-            before do
-              allow(SalesLog).to receive(:duplicate_logs).and_return(duplicate_logs)
-              post "/sales-logs/#{sales_log.id}/buyer-1-age", params:
+              before do
+                allow(SalesLog).to receive(:duplicate_logs).and_return(SalesLog.where(id: new_duplicate.id))
+                post "/sales-logs/#{sales_log.id}/#{page_id.dasherize}", params:
+              end
+
+              it "sets a new duplicate set id on both logs" do
+                sales_log.reload
+                new_duplicate.reload
+                expect(sales_log.duplicate_set_id).not_to be_nil
+                expect(sales_log.duplicate_set_id).to eql(new_duplicate.duplicate_set_id)
+              end
+
+              it "redirects to the duplicate logs page" do
+                expect(response).to redirect_to("/sales-logs/#{sales_log.id}/duplicate-logs?original_log_id=#{sales_log.id}")
+                follow_redirect!
+                expect(page).to have_content("These logs are duplicates")
+              end
             end
 
-            it "redirects to the duplicate logs page" do
-              expect(response).to redirect_to("/sales-logs/#{sales_log.id}/duplicate-logs?original_log_id=#{sales_log.id}")
-              follow_redirect!
-              expect(page).to have_content("These logs are duplicates")
+            context "with a set of other logs" do
+              let(:duplicate_set_id) { 100 }
+              let(:new_duplicates) { create_list(:sales_log, 2, duplicate_set_id:) }
+
+              before do
+                allow(SalesLog).to receive(:duplicate_logs).and_return(SalesLog.where(id: new_duplicates.pluck(:id)))
+                post "/sales-logs/#{sales_log.id}/#{page_id.dasherize}", params:
+              end
+
+              it "sets the logs duplicate set id to that of the set it is now part of" do
+                sales_log.reload
+                expect(sales_log.duplicate_set_id).to eql(duplicate_set_id)
+                new_duplicates.each do |log|
+                  log.reload
+                  expect(log.duplicate_set_id).to eql(duplicate_set_id)
+                end
+              end
+
+              it "redirects to the duplicate logs page" do
+                expect(response).to redirect_to("/sales-logs/#{sales_log.id}/duplicate-logs?original_log_id=#{sales_log.id}")
+                follow_redirect!
+                expect(page).to have_content("These logs are duplicates")
+              end
+            end
+
+            context "when the log was previously in a different duplicate set" do
+              context "with a single other log" do
+                let(:old_duplicate_set_id) { 110 }
+                let!(:old_duplicate) { create(:sales_log, duplicate_set_id: old_duplicate_set_id) }
+                let(:sales_log) { create(:sales_log, assigned_to: user, duplicate_set_id: old_duplicate_set_id) }
+                let(:new_duplicate) { create(:sales_log) }
+
+                before do
+                  allow(SalesLog).to receive(:duplicate_logs).and_return(SalesLog.where(id: new_duplicate.id))
+                  post "/sales-logs/#{sales_log.id}/#{page_id.dasherize}", params:
+                end
+
+                it "updates the relevant duplicate set ids" do
+                  sales_log.reload
+                  old_duplicate.reload
+                  new_duplicate.reload
+                  expect(old_duplicate.duplicate_set_id).to be_nil
+                  expect(sales_log.duplicate_set_id).not_to be_nil
+                  expect(sales_log.duplicate_set_id).to eql(new_duplicate.duplicate_set_id)
+                end
+              end
+
+              context "with multiple other logs" do
+                let(:old_duplicate_set_id) { 120 }
+                let!(:old_duplicates) { create_list(:sales_log, 2, duplicate_set_id: old_duplicate_set_id) }
+                let(:sales_log) { create(:sales_log, assigned_to: user, duplicate_set_id: old_duplicate_set_id) }
+                let(:new_duplicate) { create(:sales_log) }
+
+                before do
+                  allow(SalesLog).to receive(:duplicate_logs).and_return(SalesLog.where(id: new_duplicate.id))
+                  post "/sales-logs/#{sales_log.id}/#{page_id.dasherize}", params:
+                end
+
+                it "updates the relevant duplicate set ids" do
+                  sales_log.reload
+                  new_duplicate.reload
+                  old_duplicates.each do |log|
+                    log.reload
+                    expect(log.duplicate_set_id).to eql(old_duplicate_set_id)
+                  end
+                  expect(sales_log.duplicate_set_id).not_to be_nil
+                  expect(sales_log.duplicate_set_id).not_to eql(old_duplicate_set_id)
+                  expect(sales_log.duplicate_set_id).to eql(new_duplicate.duplicate_set_id)
+                end
+              end
+            end
+          end
+
+          context "when the answer makes the log stop being a duplicate" do
+            context "when the log had one duplicate" do
+              let(:old_duplicate_set_id) { 130 }
+              let!(:old_duplicate) { create(:sales_log, duplicate_set_id: old_duplicate_set_id) }
+              let(:sales_log) { create(:sales_log, assigned_to: user, duplicate_set_id: old_duplicate_set_id) }
+
+              before do
+                allow(SalesLog).to receive(:duplicate_logs).and_return(SalesLog.none)
+                post "/sales-logs/#{sales_log.id}/#{page_id.dasherize}", params:
+              end
+
+              it "updates the relevant duplicate set ids" do
+                sales_log.reload
+                old_duplicate.reload
+                expect(old_duplicate.duplicate_set_id).to be_nil
+                expect(sales_log.duplicate_set_id).to be_nil
+              end
+            end
+
+            context "when the log had multiple duplicates" do
+              let(:old_duplicate_set_id) { 140 }
+              let!(:old_duplicates) { create_list(:sales_log, 2, duplicate_set_id: old_duplicate_set_id) }
+              let(:sales_log) { create(:sales_log, assigned_to: user, duplicate_set_id: old_duplicate_set_id) }
+
+              before do
+                allow(SalesLog).to receive(:duplicate_logs).and_return(SalesLog.none)
+                post "/sales-logs/#{sales_log.id}/#{page_id.dasherize}", params:
+              end
+
+              it "updates the relevant duplicate set ids" do
+                sales_log.reload
+                old_duplicates.each do |log|
+                  log.reload
+                  expect(log.duplicate_set_id).to eql(old_duplicate_set_id)
+                end
+                expect(sales_log.duplicate_set_id).to be_nil
+              end
             end
           end
         end