diff --git a/app/main/views/organisations.py b/app/main/views/organisations.py
index b7b0d67db..c59e103ac 100644
--- a/app/main/views/organisations.py
+++ b/app/main/views/organisations.py
@@ -1,3 +1,5 @@
+from collections import OrderedDict
+
 from flask import flash, redirect, render_template, request, session, url_for
 from flask_login import current_user, login_required
 from notifications_python_client.errors import HTTPError
@@ -416,7 +418,10 @@ def edit_organisation_domains(org_id):
     if form.validate_on_submit():
         organisations_client.update_organisation(
             org_id,
-            domains=list(filter(None, form.domains.data)),
+            domains=list(OrderedDict.fromkeys(
+                domain.lower()
+                for domain in filter(None, form.domains.data)
+            )),
         )
         return redirect(url_for('.organisation_settings', org_id=org_id))
 
diff --git a/tests/app/main/views/organisations/test_organisation_invites.py b/tests/app/main/views/organisations/test_organisation_invites.py
index 2baffa786..f1bf4ee10 100644
--- a/tests/app/main/views/organisations/test_organisation_invites.py
+++ b/tests/app/main/views/organisations/test_organisation_invites.py
@@ -736,6 +736,8 @@ def test_view_organisation_domains(
     (
         {
             'domains-0': 'example.gov.uk',
+            'domains-2': 'example.gov.uk',
+            'domains-3': 'EXAMPLE.GOV.UK',
             'domains-5': 'test.gov.uk',
         },
         {