diff --git a/core/logx/writer.go b/core/logx/writer.go
index ac19aa245..4395d5a63 100644
--- a/core/logx/writer.go
+++ b/core/logx/writer.go
@@ -254,11 +254,10 @@ func (n nopWriter) Stack(_ any) {
 func (n nopWriter) Stat(_ any, _ ...LogField) {
 }
 
-func buildPlainFields(fields ...LogField) []string {
-	var items []string
-
-	for _, field := range fields {
-		items = append(items, fmt.Sprintf("%s=%+v", field.Key, field.Value))
+func buildPlainFields(fields logEntry) []string {
+	items := make([]string, 0, len(fields))
+	for k, v := range fields {
+		items = append(items, fmt.Sprintf("%s=%+v", k, v))
 	}
 
 	return items
@@ -289,15 +288,17 @@ func output(writer io.Writer, level string, val any, fields ...LogField) {
 	}
 
 	fields = combineGlobalFields(fields)
+	// +3 for timestamp, level and content
+	entry := make(logEntry, len(fields)+3)
+	for _, field := range fields {
+		entry[field.Key] = field.Value
+	}
 
 	switch atomic.LoadUint32(&encoding) {
 	case plainEncodingType:
-		writePlainAny(writer, level, val, buildPlainFields(fields...)...)
+		plainFields := buildPlainFields(entry)
+		writePlainAny(writer, level, val, plainFields...)
 	default:
-		entry := make(logEntry)
-		for _, field := range fields {
-			entry[field.Key] = field.Value
-		}
 		entry[timestampKey] = getTimestamp()
 		entry[levelKey] = level
 		entry[contentKey] = val
diff --git a/core/logx/writer_test.go b/core/logx/writer_test.go
index 0f810975f..6ac98bc1d 100644
--- a/core/logx/writer_test.go
+++ b/core/logx/writer_test.go
@@ -189,6 +189,41 @@ func TestWritePlainAny(t *testing.T) {
 	assert.Contains(t, buf.String(), "runtime/debug.Stack")
 }
 
+func TestWritePlainDuplicate(t *testing.T) {
+	old := atomic.SwapUint32(&encoding, plainEncodingType)
+	t.Cleanup(func() {
+		atomic.StoreUint32(&encoding, old)
+	})
+
+	var buf bytes.Buffer
+	output(&buf, levelInfo, "foo", LogField{
+		Key:   "first",
+		Value: "a",
+	}, LogField{
+		Key:   "first",
+		Value: "b",
+	})
+	assert.Contains(t, buf.String(), "foo")
+	assert.NotContains(t, buf.String(), "first=a")
+	assert.Contains(t, buf.String(), "first=b")
+
+	buf.Reset()
+	output(&buf, levelInfo, "foo", LogField{
+		Key:   "first",
+		Value: "a",
+	}, LogField{
+		Key:   "first",
+		Value: "b",
+	}, LogField{
+		Key:   "second",
+		Value: "c",
+	})
+	assert.Contains(t, buf.String(), "foo")
+	assert.NotContains(t, buf.String(), "first=a")
+	assert.Contains(t, buf.String(), "first=b")
+	assert.Contains(t, buf.String(), "second=c")
+}
+
 func TestLogWithLimitContentLength(t *testing.T) {
 	maxLen := atomic.LoadUint32(&maxContentLength)
 	atomic.StoreUint32(&maxContentLength, 10)