fix(schema): throw error if duplicate index definition using unique in schema path and subsequent .index() call

lib/schema.js

@@ -2134,6 +2134,12 @@ Schema.prototype.index = function(fields, options) {
     }
   }
 
+  for (const existingIndex of this.indexes()) {
+    if (util.isDeepStrictEqual(existingIndex[0], fields)) {
+      throw new MongooseError(`Schema already has an index on ${JSON.stringify(fields)}`);
+    }
+  }
+
   this._indexes.push([fields, options]);
   return this;
 };

test/schema.test.js

@@ -3278,4 +3278,21 @@
     assert.ok(subdoc instanceof mongoose.Document);
     assert.equal(subdoc.getAnswer(), 42);
   });
+  it('throws "already has an index" error if duplicate index definition (gh-15056)', function() {
+    const ObjectKeySchema = new mongoose.Schema({
+      key: {
+        type: String,
+        required: true,
+        unique: true,
+      },
+      type: {
+        type: String,
+        required: false,
+      },
+    });
+
+    assert.throws(() => {
+      ObjectKeySchema.index({ key: 1 });
+    }, /MongooseError.*already has an index/);
+  });
 });