diff --git a/server/storage/backend/tx_buffer.go b/server/storage/backend/tx_buffer.go
index 7c2f9d63ac4a..f963351cf1e9 100644
--- a/server/storage/backend/tx_buffer.go
+++ b/server/storage/backend/tx_buffer.go
@@ -16,9 +16,11 @@ package backend
 
 import (
 	"bytes"
+	"fmt"
 	"sort"
 
 	"go.etcd.io/etcd/client/pkg/v3/verify"
+	"go.etcd.io/etcd/server/v3/storage/schema"
 )
 
 const bucketBufferInitialSize = 512
@@ -83,13 +85,38 @@ func (txw *txWriteBuffer) writeback(txr *txReadBuffer) {
 		if !ok {
 			delete(txw.buckets, k)
 			txr.buckets[k] = wb
-			continue
+			rb = wb
+		} else {
+			if seq, ok := txw.bucket2seq[k]; ok && !seq && wb.used > 1 {
+				// assume no duplicate keys
+				sort.Sort(wb)
+			}
+			rb.merge(wb)
 		}
-		if seq, ok := txw.bucket2seq[k]; ok && !seq && wb.used > 1 {
-			// assume no duplicate keys
-			sort.Sort(wb)
+		if k == schema.Meta.ID() {
+			// TODO: remove the duplicated items
+			// Meta bucket is the only case which may have duplicated keys.
+		} else {
+			verifyNoDuplicatedKeys(rb)
 		}
-		rb.merge(wb)
+		// Alternative solution: only verify the Key bucket. Reasons:
+		//   1. The keys in the Key bucket are monotonically increasing
+		//      revisions, so there will never have duplicated keys. So no
+		//      need to perform the operation of removing duplicated keys
+		//      from the Key bucket. The Key bucket is the most performance
+		//      sensitive bucket, so it can also increase the performance
+		//      (Need to run benchmark the double confirm this).
+		//   2. In case we add other buckets in the future, which may break
+		//      the invariant property. Other buckets are not performance
+		//      sensitive, so we just keep them as they are for simplicity.
+		//
+		/*
+			if k == schema.Key.ID() {
+				verifyNoDuplicatedKeys(rb)
+			} else {
+				// TODO: remove the duplicated items
+			}
+		*/
 	}
 	txw.reset()
 	// increase the buffer version
@@ -205,16 +232,19 @@ func (bb *bucketBuffer) merge(bbsrc *bucketBuffer) {
 	}
 
 	sort.Stable(bb)
+}
 
-	// remove duplicates, using only newest update
-	widx := 0
-	for ridx := 1; ridx < bb.used; ridx++ {
-		if !bytes.Equal(bb.buf[ridx].key, bb.buf[widx].key) {
-			widx++
+func verifyNoDuplicatedKeys(bb *bucketBuffer) {
+	verify.Verify(func() {
+		keyMaps := make(map[string]struct{})
+		for i := 0; i < bb.used; i++ {
+			data := bb.buf[i]
+			if _, ok := keyMaps[string(data.key)]; ok {
+				panic(fmt.Sprintf("found duplicated keys in the bucketBuffer: %s", string(data.key)))
+			}
+			keyMaps[string(data.key)] = struct{}{}
 		}
-		bb.buf[widx] = bb.buf[ridx]
-	}
-	bb.used = widx + 1
+	})
 }
 
 func (bb *bucketBuffer) Len() int { return bb.used }