diff --git a/src/update/index_documents/merge_function.rs b/src/update/index_documents/merge_function.rs
index 941b17536..7f91b6716 100644
--- a/src/update/index_documents/merge_function.rs
+++ b/src/update/index_documents/merge_function.rs
@@ -70,7 +70,7 @@ pub fn documents_merge(key: &[u8], _values: &[Cow<[u8]>]) -> anyhow::Result<Vec<
     bail!("merging documents is an error ({:?})", key.as_bstr())
 }
 
-pub fn merge_two_obkv(base: obkv::KvReader, update: obkv::KvReader, buffer: &mut Vec<u8>) {
+pub fn merge_two_obkvs(base: obkv::KvReader, update: obkv::KvReader, buffer: &mut Vec<u8>) {
     use itertools::merge_join_by;
     use itertools::EitherOrBoth::{Both, Left, Right};
 
diff --git a/src/update/index_documents/mod.rs b/src/update/index_documents/mod.rs
index 807147747..10fd83f36 100644
--- a/src/update/index_documents/mod.rs
+++ b/src/update/index_documents/mod.rs
@@ -491,7 +491,7 @@ mod tests {
     use heed::EnvOpenOptions;
 
     #[test]
-    fn simple_replacement() {
+    fn simple_document_replacement() {
         let path = tempfile::tempdir().unwrap();
         let mut options = EnvOpenOptions::new();
         options.map_size(10 * 1024 * 1024); // 10 MB
@@ -516,7 +516,7 @@ mod tests {
         IndexDocuments::new(&mut wtxn, &index).execute(content, |_, _| ()).unwrap();
         wtxn.commit().unwrap();
 
-        // Check that there is **always*** 3 documents.
+        // Check that there is **always** 3 documents.
         let rtxn = index.read_txn().unwrap();
         let count = index.number_of_documents(&rtxn).unwrap();
         assert_eq!(count, 3);
@@ -528,10 +528,73 @@ mod tests {
         IndexDocuments::new(&mut wtxn, &index).execute(content, |_, _| ()).unwrap();
         wtxn.commit().unwrap();
 
-        // Check that there is **always*** 3 documents.
+        // Check that there is **always** 3 documents.
         let rtxn = index.read_txn().unwrap();
         let count = index.number_of_documents(&rtxn).unwrap();
         assert_eq!(count, 3);
         drop(rtxn);
     }
+
+    #[test]
+    fn simple_document_merge() {
+        let path = tempfile::tempdir().unwrap();
+        let mut options = EnvOpenOptions::new();
+        options.map_size(10 * 1024 * 1024); // 10 MB
+
+        let index = Index::new(options, &path).unwrap();
+
+        // First we send 3 documents with duplicate ids and
+        // change the index method to merge documents.
+        let mut wtxn = index.write_txn().unwrap();
+        let content = &b"id,name\n1,kevin\n1,kevina\n1,benoit\n"[..];
+        let mut builder = IndexDocuments::new(&mut wtxn, &index);
+        builder.index_documents_method(IndexDocumentsMethod::UpdateDocuments);
+        builder.execute(content, |_, _| ()).unwrap();
+        wtxn.commit().unwrap();
+
+        // Check that there is only 1 document now.
+        let rtxn = index.read_txn().unwrap();
+        let count = index.number_of_documents(&rtxn).unwrap();
+        assert_eq!(count, 1);
+
+        // Check that we get only one document from the database.
+        let docs = index.documents(&rtxn, Some(0)).unwrap();
+        assert_eq!(docs.len(), 1);
+        let (id, doc) = docs[0];
+        assert_eq!(id, 0);
+
+        // Check that this document is equal to the last one sent.
+        let mut doc_iter = doc.iter();
+        assert_eq!(doc_iter.next(), Some((0, &br#""1""#[..])));
+        assert_eq!(doc_iter.next(), Some((1, &br#""benoit""#[..])));
+        assert_eq!(doc_iter.next(), None);
+        drop(rtxn);
+
+        // Second we send 1 document with id 1, to force it to be merged with the previous one.
+        let mut wtxn = index.write_txn().unwrap();
+        let content = &b"id,age\n1,25\n"[..];
+        let mut builder = IndexDocuments::new(&mut wtxn, &index);
+        builder.index_documents_method(IndexDocumentsMethod::UpdateDocuments);
+        builder.execute(content, |_, _| ()).unwrap();
+        wtxn.commit().unwrap();
+
+        // Check that there is **always** 1 document.
+        let rtxn = index.read_txn().unwrap();
+        let count = index.number_of_documents(&rtxn).unwrap();
+        assert_eq!(count, 1);
+
+        // Check that we get only one document from the database.
+        let docs = index.documents(&rtxn, Some(0)).unwrap();
+        assert_eq!(docs.len(), 1);
+        let (id, doc) = docs[0];
+        assert_eq!(id, 0);
+
+        // Check that this document is equal to the last one sent.
+        let mut doc_iter = doc.iter();
+        assert_eq!(doc_iter.next(), Some((0, &br#""1""#[..])));
+        assert_eq!(doc_iter.next(), Some((1, &br#""benoit""#[..])));
+        assert_eq!(doc_iter.next(), Some((2, &br#""25""#[..])));
+        assert_eq!(doc_iter.next(), None);
+        drop(rtxn);
+    }
 }
diff --git a/src/update/index_documents/transform.rs b/src/update/index_documents/transform.rs
index 0c9425ffb..b6ec88879 100644
--- a/src/update/index_documents/transform.rs
+++ b/src/update/index_documents/transform.rs
@@ -10,7 +10,7 @@ use roaring::RoaringBitmap;
 
 use crate::{BEU32, Index, FieldsIdsMap};
 use crate::update::AvailableDocumentsIds;
-use super::merge_function::merge_two_obkv;
+use super::merge_function::merge_two_obkvs;
 use super::{create_writer, create_sorter, IndexDocumentsMethod};
 
 pub struct TransformOutput {
@@ -58,14 +58,14 @@ impl Transform<'_, '_> {
             fields_ids.push(id);
         }
 
-        /// The last value associated with an id is kept.
-        fn merge_last_win(_key: &[u8], vals: &[Cow<[u8]>]) -> anyhow::Result<Vec<u8>> {
-            vals.last().context("no last value").map(|last| last.clone().into_owned())
+        /// Only the last value associated with an id is kept.
+        fn keep_latest_obkv(_key: &[u8], obkvs: &[Cow<[u8]>]) -> anyhow::Result<Vec<u8>> {
+            obkvs.last().context("no last value").map(|last| last.clone().into_owned())
         }
 
         // We initialize the sorter with the user indexing settings.
         let mut sorter = create_sorter(
-            merge_last_win,
+            keep_latest_obkv,
             self.chunk_compression_type,
             self.chunk_compression_level,
             self.chunk_fusing_shrink_size,
@@ -132,7 +132,7 @@ impl Transform<'_, '_> {
                             let base_obkv = self.index.documents.get(&self.rtxn, &key)?
                                 .context("document not found")?;
                             let update_obkv = obkv::KvReader::new(update_obkv);
-                            merge_two_obkv(base_obkv, update_obkv, &mut obkv_buffer);
+                            merge_two_obkvs(base_obkv, update_obkv, &mut obkv_buffer);
                             (docid, obkv_buffer.as_slice())
                         }
                     }