diff --git a/core/app/models/spree/order.rb b/core/app/models/spree/order.rb
index 6f3cdd7539..b296fc0f57 100644
--- a/core/app/models/spree/order.rb
+++ b/core/app/models/spree/order.rb
@@ -151,7 +151,8 @@ def states
     validates :email, presence: true, if: :email_required?
     validates :email, "spree/email" => true, :allow_blank => true
     validates :guest_token, presence: {allow_nil: true}
-    validates :number, presence: true, uniqueness: {allow_blank: true, case_sensitive: true}
+    validates :number, presence: true
+    validates :number, uniqueness: {allow_blank: true, case_sensitive: true}, if: :number_changed?
     validates :store_id, presence: true
 
     def self.find_by_param(value)
diff --git a/core/spec/models/spree/order_spec.rb b/core/spec/models/spree/order_spec.rb
index 9ca1d93c31..448aff439c 100644
--- a/core/spec/models/spree/order_spec.rb
+++ b/core/spec/models/spree/order_spec.rb
@@ -920,6 +920,24 @@ def call
     end
   end
 
+  describe "number uniqueness" do
+    let(:existing_order) { create(:order) }
+
+    it "rejects a new order that reuses an existing number" do
+      expect(build(:order, number: existing_order.number)).not_to be_valid
+    end
+
+    it "rejects changing a persisted order's number to one already taken" do
+      order.number = existing_order.number
+      expect(order).not_to be_valid
+    end
+
+    it "skips the uniqueness query when the number is unchanged" do
+      order
+      expect { order.valid? }.not_to make_database_queries(matching: /SELECT 1 .*spree_orders.*number/i)
+    end
+  end
+
   context "#associate_user!" do
     let!(:user) { FactoryBot.create(:user) }