diff --git a/app/controllers/merge_requests_controller.rb b/app/controllers/merge_requests_controller.rb
index 11241576f..65cbbd8ba 100644
--- a/app/controllers/merge_requests_controller.rb
+++ b/app/controllers/merge_requests_controller.rb
@@ -29,13 +29,17 @@ class MergeRequestsController < ApplicationController
   def update_organisations
     if @merge_request.merging_organisation_ids
       @merge_request.merging_organisation_ids << params[:merge_request][:merging_organisation].to_i
-      @merge_request.save!
     else
-      @merge_request.update!(merging_organisation_ids: [params[:merge_request][:merging_organisation]])
+      @merge_request.merging_organisation_ids = [params[:merge_request][:merging_organisation]]
     end
     @answer_options = organisations_answer_options
     @merging_organisations_list = [@merge_request.requesting_organisation] + @merge_request.merging_organisations
-    render "organisations"
+    if @merge_request.save
+      @merging_organisations_list = [@merge_request.requesting_organisation] + @merge_request.merging_organisations
+      render "organisations"
+    else
+      render :organisations, status: :unprocessable_entity
+    end
   end
 
 private
diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index 1e0f888f6..44c588155 100644
--- a/app/models/merge_request.rb
+++ b/app/models/merge_request.rb
@@ -2,8 +2,21 @@ class MergeRequest < ApplicationRecord
   belongs_to :requesting_organisation, class_name: "Organisation"
   # has_many :merging_organisations, class_name: "Organisation", primary_key: "merging_organisation_ids", foreign_key: "id"
   # default_scope -> { select(column_names + ["merging_organisation_ids"]) }
+  validate :validate_merging_organisations
 
   def merging_organisations
     Organisation.where(id: merging_organisation_ids)
   end
+
+private
+
+  def validate_merging_organisations
+    if MergeRequest.where(requesting_organisation_id: merging_organisation_ids).count.positive?
+      errors.add(:merging_organisation, I18n.t("validations.merge_request.organisation_part_of_another_merge"))
+    end
+
+    if merging_organisation_ids&.any? { |org_id| MergeRequest.where("merging_organisation_ids @> ARRAY[?]", org_id).exists? }
+      errors.add(:merging_organisation, I18n.t("validations.merge_request.organisation_part_of_another_merge"))
+    end
+  end
 end
diff --git a/config/locales/en.yml b/config/locales/en.yml
index 074a00147..8c5858076 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -503,6 +503,8 @@ en:
       discounted_ownership_value: "Mortgage, deposit, and grant total must equal %{value_with_discount}"
       monthly_rent:
         higher_than_expected: "Basic monthly rent must be between £0.00 and £9,999.00"
+    merge_request:
+      organisation_part_of_another_merge: "This organisation is part of another merge - select a different one"
 
   soft_validations:
     net_income:
diff --git a/spec/requests/merge_requests_controller_spec.rb b/spec/requests/merge_requests_controller_spec.rb
index 66a2cbb75..b9f963278 100644
--- a/spec/requests/merge_requests_controller_spec.rb
+++ b/spec/requests/merge_requests_controller_spec.rb
@@ -2,7 +2,7 @@ require "rails_helper"
 
 RSpec.describe MergeRequestsController, type: :request do
   let(:organisation) { user.organisation }
-  let!(:other_organisation) { FactoryBot.create(:organisation) }
+  let!(:other_organisation) { FactoryBot.create(:organisation, name: "Other Test Org") }
   let(:headers) { { "Accept" => "text/html" } }
   let(:page) { Capybara::Node::Simple.new(response.body) }
   let(:user) { FactoryBot.create(:user, :data_coordinator) }
@@ -30,17 +30,51 @@ RSpec.describe MergeRequestsController, type: :request do
     describe "#update_organisations" do
       let(:params) { { merge_request: { merging_organisation: other_organisation.id } } }
 
-      before do
-        other_organisation.update!(name: "Other Test Org")
-        patch "/merge-request/#{merge_request.id}/organisations", headers:, params:
+      context "when updating a merge request with a new organisation" do
+        before do
+          patch "/merge-request/#{merge_request.id}/organisations", headers:, params:
+        end
+
+        it "updates the merge request" do
+          merge_request.reload
+          expect(merge_request.merging_organisations.count).to eq(1)
+          expect(page).to have_content("Test Org")
+          expect(page).to have_content("Other Test Org")
+          expect(page).to have_link("Remove")
+        end
       end
 
-      it "updates the merge request" do
-        merge_request.reload
-        expect(merge_request.merging_organisations.count).to eq(1)
-        expect(page).to have_content("Test Org")
-        expect(page).to have_content("Other Test Org")
-        expect(page).to have_link("Remove")
+      context "when the user selects an organisation that requested another merge" do
+        let(:params) { { merge_request: { merging_organisation: other_organisation.id } } }
+
+        before do
+          MergeRequest.create!(requesting_organisation_id: other_organisation.id)
+          patch "/merge-request/#{merge_request.id}/organisations", headers:, params:
+        end
+
+        it "does not update the merge request" do
+          merge_request.reload
+          expect(merge_request.merging_organisations.count).to eq(0)
+          expect(response).to have_http_status(:unprocessable_entity)
+          expect(page).to have_content(I18n.t("validations.merge_request.organisation_part_of_another_merge"))
+        end
+      end
+
+      context "when the user selects an organisation that is a part of another merge" do
+        let(:another_organisation) { FactoryBot.create(:organisation, name: "Other Test Org") }
+        let(:params) { { merge_request: { merging_organisation: another_organisation.id } } }
+
+        before do
+          MergeRequest.create!(requesting_organisation_id: other_organisation.id, merging_organisation_ids: [another_organisation.id])
+          patch "/merge-request/#{merge_request.id}/organisations", headers:, params:
+        end
+
+        it "does not update the merge request" do
+          merge_request.reload
+          expect(merge_request.merging_organisations.count).to eq(0)
+          expect(response).to have_http_status(:unprocessable_entity)
+          expect(page).to have_content(I18n.t("validations.merge_request.organisation_part_of_another_merge"))
+        end
       end
     end
   end