diff --git a/app/admin/service/seed.go b/app/admin/service/seed.go
index 37705693..bfb09d45 100644
--- a/app/admin/service/seed.go
+++ b/app/admin/service/seed.go
@@ -210,9 +210,23 @@ func seedMenuTree(tx *gorm.DB, appCode string, specs []seed.MenuSpec, apiRows ma
 			// the soft-delete plugin scopes deleted_at = 0 automatically on
 			// every query against models.SysMenu.
 			var existing models.SysMenu
+			found := false
 			err := tx.Where("app_code = ? AND seed_code = ?", appCode, s.Code).First(&existing).Error
 			switch {
 			case err == nil:
+				found = true
+			case errors.Is(err, gorm.ErrRecordNotFound):
+				// Nothing under the natural key. It may still be here from
+				// before seed_code existed, under the name that identified
+				// it then.
+				existing, found, err = adoptLegacyMenu(tx, appCode, s)
+				if err != nil {
+					return nil, fmt.Errorf("%q: %w", s.Code, err)
+				}
+			default:
+				return nil, fmt.Errorf("%q: checking for an existing row: %w", s.Code, err)
+			}
+			if found {
 				row, err := repairExistingMenu(tx, existing, appCode, s, parentRow, apiRows)
 				if err != nil {
 					return nil, fmt.Errorf("%q: repairing an existing row: %w", s.Code, err)
@@ -221,10 +235,6 @@ func seedMenuTree(tx *gorm.DB, appCode string, specs []seed.MenuSpec, apiRows ma
 				ids = append(ids, row.MenuId)
 				progressed = true
 				continue
-			case errors.Is(err, gorm.ErrRecordNotFound):
-				// Not written yet; fall through to create it below.
-			default:
-				return nil, fmt.Errorf("%q: checking for an existing row: %w", s.Code, err)
 			}
 
 			row := menuRowFor(appCode, s, parentRow)
@@ -567,3 +577,45 @@ func repairPaths(tx *gorm.DB, row *models.SysMenu, s seed.MenuSpec, parentRow mo
 	row.Paths = want
 	return nil
 }
+
+// adoptLegacyMenu claims a row this application wrote before sys_menu had a
+// seed_code column, so a reseed repairs it instead of inserting a second copy
+// beside it.
+//
+// 1786700008000 added the column and left it NULL on every row already there,
+// which is right for the host's own hand-placed menus - there is nothing to
+// derive one from. An application's rows are in that population too, and for
+// those the value is derivable, because menu_name is what identified them
+// before the column existed. Without this the natural-key lookup misses them,
+// the seed inserts a duplicate, and the unique index cannot object: NULL
+// never collides.
+//
+// Ambiguity is refused rather than guessed. menuName concatenates two
+// pascalCase strings and pascalCase is not injective, so two specs can land
+// on one name; picking one of several rows would attach an application's
+// menu to whichever the database returned first.
+func adoptLegacyMenu(tx *gorm.DB, appCode string, s seed.MenuSpec) (models.SysMenu, bool, error) {
+	name := menuName(appCode, s.Code)
+	var rows []models.SysMenu
+	if err := tx.Where("app_code = ? AND menu_name = ? AND seed_code IS NULL", appCode, name).
+		Find(&rows).Error; err != nil {
+		return models.SysMenu{}, false, fmt.Errorf("looking for a row written before seed_code existed: %w", err)
+	}
+	switch len(rows) {
+	case 0:
+		return models.SysMenu{}, false, nil
+	case 1:
+	default:
+		return models.SysMenu{}, false, fmt.Errorf(
+			"%d rows carry menu_name %q with no seed_code; which of them belongs to %q cannot be decided here, because menuName is not reversible - reconcile them by hand",
+			len(rows), name, s.Code)
+	}
+
+	seedCode := s.Code
+	if err := tx.Model(&models.SysMenu{}).Where("menu_id = ?", rows[0].MenuId).
+		Update("seed_code", seedCode).Error; err != nil {
+		return models.SysMenu{}, false, fmt.Errorf("claiming the row written before seed_code existed: %w", err)
+	}
+	rows[0].SeedCode = &seedCode
+	return rows[0], true, nil
+}
diff --git a/app/admin/service/seed_test.go b/app/admin/service/seed_test.go
index 9826689b..983850f3 100644
--- a/app/admin/service/seed_test.go
+++ b/app/admin/service/seed_test.go
@@ -3,6 +3,7 @@ package service
 import (
 	"context"
 	"errors"
+	"fmt"
 	"strconv"
 	"strings"
 	"sync"
@@ -1140,3 +1141,113 @@ func orderMenuSpecs(title, component string) ([]seed.MenuSpec, []seed.ApiSpec) {
 	}
 	return menus, apis
 }
+
+// 1786700008000 added seed_code and left it NULL on every row already there.
+// An application's rows are in that population, and the natural-key lookup
+// misses them, so the seed used to insert a second copy beside each one -
+// which the unique index cannot object to, because NULL never collides.
+func TestSeedMenusAdoptsARowWrittenBeforeSeedCodeExisted(t *testing.T) {
+	db := newSeedTestDB(t)
+	useCompositeSeedCodeIndex(t, db)
+	seedAdminRole(t, db)
+
+	// What an older SeedMenus left: app_code set, seed_code absent, and the
+	// name that identified it then.
+	legacy := models.SysMenu{
+		MenuName: menuName("order", "dir"), AppCode: "order", Title: "the old title",
+		MenuType: contractmodels.Directory, Path: "/apps/order", Component: "Layout", Sort: 10,
+	}
+	if err := db.Create(&legacy).Error; err != nil {
+		t.Fatal(err)
+	}
+
+	menus, apis := orderMenuSpecs("Orders", "apps/order/index")
+	if err := (adminSeeder{}).SeedMenus(db, "order", menus, apis); err != nil {
+		t.Fatalf("seed: %v", err)
+	}
+
+	var rows []models.SysMenu
+	if err := db.Where("app_code = ? AND menu_name = ?", "order", menuName("order", "dir")).
+		Find(&rows).Error; err != nil {
+		t.Fatal(err)
+	}
+	if len(rows) != 1 {
+		t.Fatalf("%d rows carry that name; the row from before the column existed was not found", len(rows))
+	}
+	if rows[0].MenuId != legacy.MenuId {
+		t.Errorf("menu_id = %d, want the row that was already there (%d)", rows[0].MenuId, legacy.MenuId)
+	}
+	if rows[0].SeedCode == nil || *rows[0].SeedCode != "dir" {
+		t.Errorf("seed_code = %v, want it claimed", rows[0].SeedCode)
+	}
+	// Adopted and then repaired, like any other existing row.
+	if rows[0].Title != "Order Example" {
+		t.Errorf("title = %q; the adopted row was not brought up to the spec", rows[0].Title)
+	}
+}
+
+// menuName concatenates two pascalCase strings and pascalCase is not
+// injective, so two specs can land on one name. Picking one of several rows
+// would attach an application's menu to whichever the database returned
+// first.
+func TestSeedMenusRefusesAnAmbiguousAdoption(t *testing.T) {
+	db := newSeedTestDB(t)
+	useCompositeSeedCodeIndex(t, db)
+	seedAdminRole(t, db)
+
+	for i := 0; i < 2; i++ {
+		row := models.SysMenu{
+			MenuName: menuName("order", "dir"), AppCode: "order", Title: fmt.Sprintf("copy %d", i),
+			MenuType: contractmodels.Directory,
+		}
+		if err := db.Create(&row).Error; err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	menus, apis := orderMenuSpecs("Orders", "apps/order/index")
+	err := (adminSeeder{}).SeedMenus(db, "order", menus, apis)
+	if err == nil {
+		t.Fatal("an ambiguous adoption was accepted")
+	}
+	if !strings.Contains(err.Error(), "2 rows") || !strings.Contains(err.Error(), "by hand") {
+		t.Errorf("error = %q, it has to say how many and that it is not deciding", err)
+	}
+	// And it did not write a third.
+	var n int64
+	db.Model(&models.SysMenu{}).Where("app_code = ? AND menu_name = ?", "order", menuName("order", "dir")).Count(&n)
+	if n != 2 {
+		t.Errorf("%d rows carry that name; the refusal still inserted", n)
+	}
+}
+
+// A row belonging to another application, or to the host, carries a different
+// app_code and is not this application's to claim.
+func TestSeedMenusDoesNotAdoptAnotherApplicationsRow(t *testing.T) {
+	db := newSeedTestDB(t)
+	useCompositeSeedCodeIndex(t, db)
+	seedAdminRole(t, db)
+
+	other := models.SysMenu{
+		MenuName: menuName("order", "dir"), AppCode: "crm", Title: "crm's own",
+		MenuType: contractmodels.Directory,
+	}
+	if err := db.Create(&other).Error; err != nil {
+		t.Fatal(err)
+	}
+
+	menus, apis := orderMenuSpecs("Orders", "apps/order/index")
+	if err := (adminSeeder{}).SeedMenus(db, "order", menus, apis); err != nil {
+		t.Fatalf("seed: %v", err)
+	}
+
+	var after models.SysMenu
+	db.Where("menu_id = ?", other.MenuId).First(&after)
+	if after.SeedCode != nil || after.Title != "crm's own" {
+		t.Errorf("another application's row was claimed: %+v", after)
+	}
+	var mine models.SysMenu
+	if err := db.Where("app_code = ? AND seed_code = ?", "order", "dir").First(&mine).Error; err != nil {
+		t.Fatalf("this application's own row was not created: %v", err)
+	}
+}