diff --git a/logger/logger.go b/logger/logger.go
index af4b0e8..d538a9b 100644
--- a/logger/logger.go
+++ b/logger/logger.go
@@ -62,8 +62,17 @@ func LevelOption(level logger.LogLevel) Option {
 
 // slogLogger implements logger.Logger backed by log/slog.
 type slogLogger struct {
+	// base is the pristine slog.Logger before any WithFields attrs are
+	// applied. It is used to rebuild logger after each WithFields call.
+	base *slog.Logger
+	// logger is the effective slog.Logger used for emitting entries.
 	logger *slog.Logger
-	level  *slog.LevelVar
+	// args holds the accumulated WithFields key-value pairs in insertion
+	// order, deduplicated by key (last write wins). slog.Logger.With does not
+	// deduplicate, so chaining WithFields with a repeated key would otherwise
+	// emit duplicate JSON keys.
+	args  []any
+	level *slog.LevelVar
 }
 
 // NewLogger creates a new logger.Logger backed by log/slog.
@@ -90,6 +99,7 @@ func NewLogger(opts ...Option) logger.Logger {
 	}
 
 	return &slogLogger{
+		base:   sl,
 		logger: sl,
 		level:  levelVar,
 	}
@@ -186,18 +196,38 @@ func revertLevel(lvl slog.Level) logger.LogLevel {
 }
 
 // WithFields returns a new Logger with the given fields attached to every
-// subsequent log entry.
+// subsequent log entry. A field whose key was already set by an earlier
+// WithFields call is overwritten (last write wins) rather than duplicated.
 func (l *slogLogger) WithFields(fields map[string]any) logger.Logger {
-	attrs := make([]any, 0, len(fields)*2)
-	for k, v := range fields {
-		attrs = append(attrs, k, v)
-	}
+	args := mergeArgs(l.args, fields)
 	return &slogLogger{
-		logger: l.logger.With(attrs...),
+		base:   l.base,
+		logger: l.base.With(args...),
+		args:   args,
 		level:  l.level,
 	}
 }
 
+// mergeArgs merges fields into args (a flat [key, value, ...] slice),
+// overwriting any existing key in place to preserve insertion order.
+func mergeArgs(args []any, fields map[string]any) []any {
+	index := make(map[string]int, len(args)/2+len(fields))
+	out := make([]any, 0, len(args)+len(fields)*2)
+	for i := 0; i < len(args); i += 2 {
+		index[args[i].(string)] = len(out)
+		out = append(out, args[i], args[i+1])
+	}
+	for k, v := range fields {
+		if i, ok := index[k]; ok {
+			out[i+1] = v
+		} else {
+			index[k] = len(out)
+			out = append(out, k, v)
+		}
+	}
+	return out
+}
+
 // Trace logs at Trace level.
 func (l *slogLogger) Trace(args ...any) {
 	l.log(levelTrace, args...)
diff --git a/logger/logger_test.go b/logger/logger_test.go
index 4b76381..2c95018 100644
--- a/logger/logger_test.go
+++ b/logger/logger_test.go
@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"log/slog"
 	"regexp"
+	"strings"
 	"testing"
 	"time"
 
@@ -184,6 +185,25 @@ func TestWithFields(t *testing.T) {
 	}
 }
 
+func TestWithFieldsDeduplicatesKeys(t *testing.T) {
+	var buf bytes.Buffer
+	l := NewLogger(
+		OutputOption(&buf),
+		LevelOption(logger.InfoLevel),
+		FormatOption(logger.JSONFormat),
+	)
+	l = l.WithFields(map[string]any{"kind": "service", "service": "s"}).WithFields(map[string]any{"kind": "handler"})
+	l.Info("hello")
+	out := buf.String()
+	// "kind" must appear exactly once, with the last value winning.
+	if strings.Count(out, `"kind"`) != 1 {
+		t.Errorf("expected one %q key, got %q", "kind", out)
+	}
+	if !strings.Contains(out, `"kind":"handler"`) {
+		t.Errorf("expected kind=handler, got %q", out)
+	}
+}
+
 func TestIsLevelEnabled(t *testing.T) {
 	l := NewLogger(LevelOption(logger.InfoLevel))
 	if !l.IsLevelEnabled(logger.WarnLevel) {