From e2ebc754052ef32b12b6c8cbc55268e2dad98975 Mon Sep 17 00:00:00 2001
From: Valeri Karpov <val@karpov.io>
Date: Fri, 13 Dec 2024 11:20:10 -0500
Subject: [PATCH 1/2] fix(schema): throw error if duplicate index definition
 using `unique` in schema path and subsequent `.index()` call

Fix #15056
---
 lib/schema.js       |  6 ++++++
 test/schema.test.js | 17 +++++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/lib/schema.js b/lib/schema.js
index 7caaac75920..26ea5d94818 100644
--- a/lib/schema.js
+++ b/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;
 };
diff --git a/test/schema.test.js b/test/schema.test.js
index 3076c9df62a..58f761e0219 100644
--- a/test/schema.test.js
+++ b/test/schema.test.js
@@ -3278,4 +3278,21 @@ describe('schema', function() {
     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/);
+  });
 });

From f70e844c07ca1e56fcebb219bbc2eeab18975d27 Mon Sep 17 00:00:00 2001
From: Valeri Karpov <val@karpov.io>
Date: Mon, 16 Dec 2024 11:04:45 -0500
Subject: [PATCH 2/2] style: fix lint

---
 test/schema.test.js | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/test/schema.test.js b/test/schema.test.js
index 58f761e0219..6378ec340ac 100644
--- a/test/schema.test.js
+++ b/test/schema.test.js
@@ -3283,12 +3283,12 @@ describe('schema', function() {
       key: {
         type: String,
         required: true,
-        unique: true,
+        unique: true
       },
       type: {
         type: String,
-        required: false,
-      },
+        required: false
+      }
     });
 
     assert.throws(() => {