fix: ensures no duplicate tax lines when completing cart (#1262)

* fix: ensures that no duplicate tax lines are created when completing cart

integration-tests/api/__tests__/taxes/orders.js

@@ -197,6 +197,111 @@ describe("Order Taxes", () => {
     expect(response.data.order.total).toEqual(2300)
   })
 
+  test("completing cart with failure doesn't duplicate", async () => {
+    const product1 = await simpleProductFactory(
+      dbConnection,
+      {
+        variants: [
+          {
+            id: "test-variant",
+          },
+        ],
+      },
+      100
+    )
+
+    const product2 = await simpleProductFactory(
+      dbConnection,
+      {
+        variants: [
+          {
+            id: "test-variant-2",
+          },
+        ],
+      },
+      100
+    )
+
+    const region = await simpleRegionFactory(dbConnection, {
+      name: "Test region",
+      tax_rate: 12,
+    })
+
+    await simpleProductTaxRateFactory(dbConnection, {
+      product_id: product1.id,
+      rate: {
+        region_id: region.id,
+        rate: 25,
+      },
+    })
+
+    await simpleProductTaxRateFactory(dbConnection, {
+      product_id: product2.id,
+      rate: {
+        region_id: region.id,
+        rate: 20,
+      },
+    })
+
+    const cart = await simpleCartFactory(
+      dbConnection,
+      {
+        region: region.id,
+        email: "test@testson.com",
+        line_items: [
+          {
+            variant_id: "test-variant",
+            unit_price: 100,
+          },
+          {
+            variant_id: "test-variant-2",
+            unit_price: 50,
+          },
+        ],
+      },
+      100
+    )
+
+    const api = useApi()
+
+    await api.post(`/store/carts/${cart.id}`, {
+      email: "test@testson.com",
+    })
+
+    const failedComplete = await api
+      .post(`/store/carts/${cart.id}/complete`)
+      .catch((err) => err.response)
+
+    expect(failedComplete.status).toEqual(400)
+    expect(failedComplete.data.message).toEqual(
+      "You cannot complete a cart without a payment session."
+    )
+
+    await api.post(`/store/carts/${cart.id}/payment-sessions`)
+    const response = await api.post(`/store/carts/${cart.id}/complete`)
+
+    expect(response.status).toEqual(200)
+
+    expect(response.data.type).toEqual("order")
+    expect(response.data.data.tax_total).toEqual(35)
+    expect(response.data.data.total).toEqual(185)
+
+    expect(
+      response.data.data.items.flatMap((li) => li.tax_lines).length
+    ).toEqual(2)
+
+    expect(response.data.data.items[0].tax_lines).toEqual([
+      expect.objectContaining({
+        rate: 25,
+      }),
+    ])
+    expect(response.data.data.items[1].tax_lines).toEqual([
+      expect.objectContaining({
+        rate: 20,
+      }),
+    ])
+  })
+
   test("completing cart creates tax lines", async () => {
     const product1 = await simpleProductFactory(
       dbConnection,