Fix accessors being seen as duplicates of each other

If an object has a getter and setter of the same name, then they
shouldn't be treated as duplicate properties.

packages/babel-plugin-transform-es2015-duplicate-keys/src/index.js

@@ -14,13 +14,45 @@ export default function() {
         const { node } = path;
         const plainProps = node.properties.filter((prop) => !t.isSpreadProperty(prop) && !prop.computed);
 
-        const alreadySeenNames = Object.create(null);
+        // A property is a duplicate key if:
+        // * the property is a data property, and is preceeded by a data,
+        //   getter, or setter property of the same name.
+        // * the property is a getter property, and is preceeded by a data or
+        //   getter property of the same name.
+        // * the property is a setter property, and is preceeded by a data or
+        //   setter property of the same name.
+
+        const alreadySeenData = Object.create(null);
+        const alreadySeenGetters = Object.create(null);
+        const alreadySeenSetters = Object.create(null);
 
         for (let prop of plainProps) {
           const name = getName(prop.key);
-          if (!alreadySeenNames[name]) {
-            alreadySeenNames[name] = true;
-          } else {
+          let isDuplicate = false;
+          switch (prop.kind) {
+          case "get":
+            if (alreadySeenData[name] || alreadySeenGetters[name]) {
+              isDuplicate = true;
+            } else {
+              alreadySeenGetters[name] = true;
+            }
+            break;
+          case "set":
+            if (alreadySeenData[name] || alreadySeenSetters[name]) {
+              isDuplicate = true;
+            } else {
+              alreadySeenSetters[name] = true;
+            }
+            break;
+          default:
+            if (alreadySeenData[name] || alreadySeenGetters[name] || alreadySeenSetters[name]) {
+              isDuplicate = true;
+            } else {
+              alreadySeenData[name] = true;
+            }
+          }
+
+          if (isDuplicate) {
             // Rely on the computed properties transform to split the property
             // assignment out of the object literal.
             prop.computed = true;

packages/babel-plugin-transform-es2015-duplicate-keys/test/fixtures/duplicate-keys/getters-and-setters/actual.js

@@ -0,0 +1,10 @@
+var x = {
+  get a() {},
+  set a(x) {},
+  get a() {},
+  set a(x) {},
+  a: 3,
+  b: 4,
+  get b() {},
+  set b(x) {}
+};

packages/babel-plugin-transform-es2015-duplicate-keys/test/fixtures/duplicate-keys/getters-and-setters/expected.js

@@ -0,0 +1,10 @@
+var x = {
+  get a() {},
+  set a(x) {},
+  get ["a"]() {},
+  set ["a"](x) {},
+  ["a"]: 3,
+  b: 4,
+  get ["b"]() {},
+  set ["b"](x) {}
+};