diff --git a/accounts/forms.py b/accounts/forms.py
index 30f4d8cc..5db2cb54 100644
--- a/accounts/forms.py
+++ b/accounts/forms.py
@@ -34,7 +34,18 @@ class UserInstanceForm(ModelForm):
         fields = "__all__"
 
 
-class ProfileForm(ModelForm):
+class UniqueEmailMixin:
+    """"Lost OTP?" mails only an address that is one user's: a second user
+    with the same address would switch it off for both."""
+
+    def clean_email(self):
+        email = self.cleaned_data["email"]
+        if email and get_user_model().objects.filter(email__iexact=email).exclude(pk=self.instance.pk).exists():
+            raise ValidationError(_("Another user has this email address"))
+        return email
+
+
+class ProfileForm(UniqueEmailMixin, ModelForm):
     class Meta:
         model = get_user_model()
         fields = ("first_name", "last_name", "email")
diff --git a/accounts/tests/test_unique_email.py b/accounts/tests/test_unique_email.py
new file mode 100644
index 00000000..610bf535
--- /dev/null
+++ b/accounts/tests/test_unique_email.py
@@ -0,0 +1,31 @@
+"""An email address belongs to one user: "Lost OTP?" mails only an exact,
+single match, so a second user with the same address would switch it off for both."""
+
+from django.contrib.auth.models import User
+from django.test import TestCase
+from django.urls import reverse
+
+from accounts.forms import ProfileForm
+from admin.forms import UserForm
+
+
+class UniqueEmailTests(TestCase):
+    def setUp(self):
+        self.alice = User.objects.create_user("alice", "alice@example.com", "pw")
+        self.bob = User.objects.create_user("bob", "", "pw")
+
+    def test_the_profile_refuses_another_users_address(self):
+        self.client.force_login(self.bob)
+        response = self.client.post(reverse("accounts:profile"), {"email": "Alice@Example.com"})
+        self.assertContains(response, "Another user has this email address")
+        self.bob.refresh_from_db()
+        self.assertEqual(self.bob.email, "")
+
+    def test_the_admin_form_refuses_another_users_address(self):
+        form = UserForm({"username": "bob", "email": "alice@example.com", "is_active": "on"}, instance=self.bob)
+        self.assertEqual(form.errors["email"], ["Another user has this email address"])
+
+    def test_own_address_and_no_address_are_kept(self):
+        User.objects.create_user("carol", "", "pw")
+        self.assertTrue(ProfileForm({"email": ""}, instance=self.bob).is_valid())
+        self.assertTrue(ProfileForm({"email": "alice@example.com"}, instance=self.alice).is_valid())
diff --git a/admin/forms.py b/admin/forms.py
index 0f9f357e..8d4f3639 100644
--- a/admin/forms.py
+++ b/admin/forms.py
@@ -5,6 +5,7 @@ from django.contrib.auth.models import Group, User
 from django.core.exceptions import ValidationError
 from django.utils.translation import gettext_lazy as _
 
+from accounts.forms import UniqueEmailMixin
 from accounts.models import UserAttributes
 
 from .models import Permission
@@ -42,7 +43,7 @@ class GroupForm(forms.ModelForm):
         fields = "__all__"
 
 
-class UserForm(forms.ModelForm):
+class UserForm(UniqueEmailMixin, forms.ModelForm):
     user_permissions = forms.ModelMultipleChoiceField(
         widget=forms.CheckboxSelectMultiple,
         queryset=Permission.objects.filter(content_type__model="permissionset"),