diff --git a/ormar/relations/relation_proxy.py b/ormar/relations/relation_proxy.py
index 7efaf41..9b90199 100644
--- a/ormar/relations/relation_proxy.py
+++ b/ormar/relations/relation_proxy.py
@@ -325,7 +325,7 @@ class RelationProxy(Generic[T], List[T]):
         :param item: child to add to relation
         :type item: Model
         """
-        new_idx = len(self)
+        new_idx = len(self) if item not in self else self.index(item)
         relation_name = self.related_field_name
         await self._owner.signals.pre_relation_add.send(
             sender=self._owner.__class__,
diff --git a/tests/test_hashes/__init__.py b/tests/test_hashes/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/tests/test_hashes/test_many_to_many.py b/tests/test_hashes/test_many_to_many.py
new file mode 100644
index 0000000..2041701
--- /dev/null
+++ b/tests/test_hashes/test_many_to_many.py
@@ -0,0 +1,100 @@
+import sqlite3
+from typing import ForwardRef, List, Optional
+
+import asyncpg
+import ormar
+import pymysql
+import pytest
+import pytest_asyncio
+
+from tests.lifespan import init_tests
+from tests.settings import create_config
+
+base_ormar_config = create_config(force_rollback=True)
+
+
+class Category(ormar.Model):
+    ormar_config = base_ormar_config.copy(tablename="categories")
+
+    id: int = ormar.Integer(primary_key=True)
+    name: str = ormar.String(max_length=40)
+
+
+class Author(ormar.Model):
+    ormar_config = base_ormar_config.copy(tablename="authors")
+
+    id: int = ormar.Integer(primary_key=True)
+    name: str = ormar.String(max_length=40)
+
+
+class Post(ormar.Model):
+    ormar_config = base_ormar_config.copy(tablename="posts")
+
+    id: int = ormar.Integer(primary_key=True)
+    title: str = ormar.String(max_length=200)
+    categories: Optional[List[Category]] = ormar.ManyToMany(Category)
+    authors: Optional[List[Author]] = ormar.ManyToMany(
+        Author, through=ForwardRef("AuthorXPosts")
+    )
+
+
+class AuthorXPosts(ormar.Model):
+    ormar_config = base_ormar_config.copy(
+        tablename="authors_x_posts", constraints=[ormar.UniqueColumns("author", "post")]
+    )
+    id: int = ormar.Integer(primary_key=True)
+    author: Optional[int] = ormar.Integer(default=None)
+    post: Optional[int] = ormar.Integer(default=None)
+
+
+Post.update_forward_refs()
+
+
+create_test_database = init_tests(base_ormar_config)
+
+
+@pytest_asyncio.fixture(scope="function", autouse=True)
+async def cleanup():
+    yield
+    async with base_ormar_config.database:
+        await Post.ormar_config.model_fields["categories"].through.objects.delete(
+            each=True
+        )
+        await Post.ormar_config.model_fields["authors"].through.objects.delete(
+            each=True
+        )
+        await Post.objects.delete(each=True)
+        await Category.objects.delete(each=True)
+        await Author.objects.delete(each=True)
+
+
+@pytest.mark.asyncio
+async def test_adding_same_m2m_model_twice():
+    async with base_ormar_config.database:
+        async with base_ormar_config.database:
+            post = await Post.objects.create(title="Hello, M2M")
+            news = await Category(name="News").save()
+
+            await post.categories.add(news)
+            await post.categories.add(news)
+
+            categories = await post.categories.all()
+            assert categories == [news]
+
+
+@pytest.mark.asyncio
+async def test_adding_same_m2m_model_twice_with_unique():
+    async with base_ormar_config.database:
+        async with base_ormar_config.database:
+            post = await Post.objects.create(title="Hello, M2M")
+            redactor = await Author(name="News").save()
+
+            await post.authors.add(redactor)
+            with pytest.raises(
+                (
+                    sqlite3.IntegrityError,
+                    pymysql.IntegrityError,
+                    asyncpg.exceptions.UniqueViolationError,
+                )
+            ):
+                await post.authors.add(redactor)