diff --git a/docs/backend/users-and-permissions.md b/docs/backend/users-and-permissions.md
index 801a08b..a2f196c 100644
--- a/docs/backend/users-and-permissions.md
+++ b/docs/backend/users-and-permissions.md
@@ -6,7 +6,7 @@ order: 50
 ---
 # Users and permissions
 
-The admin keeps WinterCMS's backend user model: the `backend_users` and `backend_user_roles` tables, roles with permission grants, and superusers who pass every check. [cabana](../../modules/cabana/README.md) signs administrators in and checks their permissions; the tables are created by the framework migrations that `migrate` runs.
+The admin keeps WinterCMS's backend user model: the `backend_users` and `backend_user_roles` tables, roles with permission grants, and superusers who pass every check. [cabana](../../modules/cabana/README.md) signs administrators in and checks their permissions; the tables are created by the framework migrations that `migrate` runs. Administrator emails are unique regardless of case. If a table copied from WinterCMS holds two emails that differ only in case, `migrate` stops and names their logins; change or remove one of them, then run `migrate` again.
 
 ## Signing in
 
diff --git a/modules/lagoon/README.md b/modules/lagoon/README.md
index d83acb1..0c7db02 100644
--- a/modules/lagoon/README.md
+++ b/modules/lagoon/README.md
@@ -121,7 +121,7 @@ func (p *Plugin) Migrations() []*gormigrate.Migration {
 | `lagoon.RollbackLast` | Rolls back the last migration of one plugin. |
 | `lagoon.Status` | Lists applied migration IDs per plugin as `lagoon.StatusRow` values. |
 | `lagoon.HistoryTableName` | Returns the gormigrate history table for a plugin ID. |
-| `lagoon.BackendAdminMigrations` | Creates the backend user, role and admin token blacklist tables and seeds the system roles. |
+| `lagoon.BackendAdminMigrations` | Creates the backend user, role and admin token blacklist tables, seeds the system roles and adds a case-insensitive unique index on `backend_users.email`. The index migration refuses to run while emails differ only in case and names the clashing logins. |
 | `lagoon.QueueMigrations` | Migrates River's schema to `lagoon.RiverSchemaVersion` and creates the `summer_jobs` job record table. |
 | `lagoon.QueueHistoryID` | History id of the job-queue set, `summercms.conga`. |
 | `lagoon.JobsTable` | Name of the job record table, `summer_jobs`. |
diff --git a/modules/lagoon/backend_admin_migrations.go b/modules/lagoon/backend_admin_migrations.go
index 75ba5cc..229cf45 100644
--- a/modules/lagoon/backend_admin_migrations.go
+++ b/modules/lagoon/backend_admin_migrations.go
@@ -1,6 +1,9 @@
 package lagoon
 
 import (
+	"fmt"
+	"strings"
+
 	"github.com/go-gormigrate/gormigrate/v2"
 	"gorm.io/gorm"
 )
@@ -9,10 +12,11 @@ import (
 // It is distinct from the frontend jwt_blacklist table.
 const BackendJWTBlacklistTable = "backend_jwt_blacklist"
 
-// BackendAdminMigrations creates Winter-shaped backend identity tables and
-// seeds the developer and publisher system roles. History is isolated under
-// the summercms.cabana plugin id. DDL is re-runnable so the system-role seed
-// stays idempotent if the history row is removed.
+// BackendAdminMigrations creates Winter-shaped backend identity tables, seeds
+// the developer and publisher system roles and makes backend user emails
+// unique case-insensitively. History is isolated under the summercms.cabana
+// plugin id. DDL is re-runnable so the system-role seed stays idempotent if
+// the history row is removed.
 var BackendAdminMigrations = []*gormigrate.Migration{
 	{
 		ID: "202609240001_backend_admin_identity",
@@ -85,4 +89,32 @@ ON CONFLICT (name) DO NOTHING`,
 			return nil
 		},
 	},
+	{
+		// Emails are matched case-insensitively at login and by admin:create,
+		// so the table enforces the same rule. Rows copied from Winter may
+		// hold two emails that differ only in case; the migration refuses to
+		// run and names them instead of picking an account to drop.
+		ID: "202610010001_backend_users_email_ci_unique",
+		Migrate: func(tx *gorm.DB) error {
+			var dupes []struct {
+				Email  string
+				Logins string
+			}
+			if err := tx.Raw(`SELECT lower(email) AS email, string_agg(login, ', ' ORDER BY id) AS logins
+FROM backend_users GROUP BY lower(email) HAVING count(*) > 1 ORDER BY 1`).Scan(&dupes).Error; err != nil {
+				return err
+			}
+			if len(dupes) > 0 {
+				parts := make([]string, len(dupes))
+				for i, d := range dupes {
+					parts[i] = fmt.Sprintf("%s (logins: %s)", d.Email, d.Logins)
+				}
+				return fmt.Errorf("lagoon: backend_users has emails that differ only in case: %s; change or remove the duplicates, then run migrate again", strings.Join(parts, "; "))
+			}
+			return tx.Exec(`CREATE UNIQUE INDEX IF NOT EXISTS backend_users_email_lower_unique ON backend_users (lower(email))`).Error
+		},
+		Rollback: func(tx *gorm.DB) error {
+			return tx.Exec(`DROP INDEX IF EXISTS backend_users_email_lower_unique`).Error
+		},
+	},
 }
diff --git a/modules/lagoon/backend_admin_migrations_test.go b/modules/lagoon/backend_admin_migrations_test.go
index fef4c72..c460141 100644
--- a/modules/lagoon/backend_admin_migrations_test.go
+++ b/modules/lagoon/backend_admin_migrations_test.go
@@ -51,9 +51,7 @@ func TestPhase09MigrationsFreshRollback(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	if err := admin.RollbackLast(); err != nil {
-		t.Fatal(err)
-	}
+	rollbackBackendAdmin(t, admin)
 	for _, name := range []string{"backend_users", "backend_user_roles", "backend_jwt_blacklist"} {
 		if gdb.Migrator().HasTable(name) {
 			t.Fatalf("%s survived admin rollback", name)
@@ -205,9 +203,7 @@ func TestBackendAdminRollback(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	if err := m.RollbackLast(); err != nil {
-		t.Fatal(err)
-	}
+	rollbackBackendAdmin(t, m)
 	for _, name := range []string{"backend_users", "backend_user_roles", "backend_jwt_blacklist"} {
 		if gdb.Migrator().HasTable(name) {
 			t.Fatalf("%s survived admin rollback", name)
@@ -443,3 +439,71 @@ func assertNoAutoMigrate(t *testing.T) {
 		}
 	}
 }
+
+// rollbackBackendAdmin rolls back every BackendAdminMigrations entry, newest
+// first, so the identity tables are dropped as a set.
+func rollbackBackendAdmin(t *testing.T, m *gormigrate.Gormigrate) {
+	t.Helper()
+	for range BackendAdminMigrations {
+		if err := m.RollbackLast(); err != nil {
+			t.Fatal(err)
+		}
+	}
+}
+
+func TestBackendAdminEmailCaseInsensitiveUnique(t *testing.T) {
+	db, _ := dedicatedDB(t, "lagoon_admin_email_ci")
+	gdb, err := Use(t.Context(), db)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := Migrate(gdb, nil); err != nil {
+		t.Fatal(err)
+	}
+	if err := gdb.Exec(`INSERT INTO backend_users (login, email, password) VALUES ('ada', 'Ada@Example.Test', 'x')`).Error; err != nil {
+		t.Fatal(err)
+	}
+	if err := gdb.Exec(`INSERT INTO backend_users (login, email, password) VALUES ('ada-2', 'ada@example.test', 'x')`).Error; err == nil {
+		t.Fatal("an email differing only in case was accepted")
+	}
+}
+
+func TestBackendAdminEmailIndexRefusesCaseDuplicates(t *testing.T) {
+	db, _ := dedicatedDB(t, "lagoon_admin_email_dupes")
+	gdb, err := Use(t.Context(), db)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := Migrate(gdb, nil); err != nil {
+		t.Fatal(err)
+	}
+	m, err := migrator(gdb, "summercms.cabana", BackendAdminMigrations)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := m.RollbackLast(); err != nil {
+		t.Fatal(err)
+	}
+	// A Winter copy can hold emails that differ only in case.
+	for _, row := range [][2]string{{"ada", "Ada@Example.Test"}, {"ada-old", "ada@example.test"}} {
+		if err := gdb.Exec(`INSERT INTO backend_users (login, email, password) VALUES (?, ?, 'x')`, row[0], row[1]).Error; err != nil {
+			t.Fatal(err)
+		}
+	}
+	err = Migrate(gdb, nil)
+	if err == nil {
+		t.Fatal("migrate created the unique email index over case duplicates")
+	}
+	if !strings.Contains(err.Error(), "ada@example.test (logins: ada, ada-old)") {
+		t.Fatalf("error does not name the duplicates: %v", err)
+	}
+	if err := gdb.Exec(`UPDATE backend_users SET email = 'ada.old@example.test' WHERE login = 'ada-old'`).Error; err != nil {
+		t.Fatal(err)
+	}
+	if err := Migrate(gdb, nil); err != nil {
+		t.Fatalf("migrate after resolving duplicates: %v", err)
+	}
+	if !indexOn(indexDefs(t, gdb, "backend_users"), "lower(email)") {
+		t.Fatal("unique lower(email) index missing")
+	}
+}