diff --git a/config/headers_test.go b/config/headers_test.go
index daac9808..f2c3ad2b 100644
--- a/config/headers_test.go
+++ b/config/headers_test.go
@@ -69,6 +69,35 @@ func TestHeadersRoundTripperSameHost(t *testing.T) {
 	}
 }
 
+func TestHeadersRoundTripperReusedRequest(t *testing.T) {
+	// The round tripper must not mutate the request it is given: reusing the
+	// same request must not accumulate another copy of every header.
+	var received []string
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		received = r.Header.Values("X-Custom-Header")
+		fmt.Fprint(w, "ok")
+	}))
+	t.Cleanup(server.Close)
+
+	headers := &Headers{
+		Headers: map[string]Header{
+			"X-Custom-Header": {Values: []string{"testvalue"}},
+		},
+	}
+	rt := NewHeadersRoundTripper(headers, http.DefaultTransport)
+
+	req, err := http.NewRequest(http.MethodGet, server.URL, nil)
+	require.NoError(t, err)
+
+	for i := range 3 {
+		resp, err := rt.RoundTrip(req)
+		require.NoError(t, err)
+		resp.Body.Close()
+		require.Equalf(t, []string{"testvalue"}, received, "header duplicated on request %d", i+1)
+		require.Emptyf(t, req.Header.Values("X-Custom-Header"), "the caller's request was modified")
+	}
+}
+
 func TestHeadersRoundTripperCrossHostRedirect(t *testing.T) {
 	// Cookie must be set on the initial request but stripped on cross-host redirects.
 	cookieOnRedirect := ""
diff --git a/config/http_config.go b/config/http_config.go
index d633479c..35a90fca 100644
--- a/config/http_config.go
+++ b/config/http_config.go
@@ -22,7 +22,6 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
-	"maps"
 	"net"
 	"net/http"
 	"net/url"
@@ -1240,8 +1239,13 @@ func cloneRequest(r *http.Request) *http.Request {
 	// Shallow copy of the struct.
 	r2 := new(http.Request)
 	*r2 = *r
-	// Deep copy of the Header.
-	maps.Copy(r.Header, r2.Header)
+	// Deep copy of the Header. The shallow copy above leaves r2.Header
+	// aliasing r.Header, so without this every round tripper that adds a
+	// header would mutate the caller's request.
+	r2.Header = r.Header.Clone()
+	if r2.Header == nil {
+		r2.Header = make(http.Header)
+	}
 	return r2
 }
 
diff --git a/config/http_config_test.go b/config/http_config_test.go
index 4d61ae12..9b3de4df 100644
--- a/config/http_config_test.go
+++ b/config/http_config_test.go
@@ -2749,3 +2749,27 @@ func TestLoadHTTPConfigFileResolvesPathsRelativeToConfigFile(t *testing.T) {
 	_, err = client.Get(ts.URL)
 	require.NoErrorf(t, err, "can't fetch URL: %v", err)
 }
+
+func TestCloneRequest(t *testing.T) {
+	t.Run("clone does not share the caller's header", func(t *testing.T) {
+		r, err := http.NewRequest(http.MethodGet, "http://example.com", nil)
+		require.NoError(t, err)
+		r.Header.Set("X-Original", "value")
+
+		r2 := cloneRequest(r)
+		r2.Header.Add("X-Added", "value")
+
+		require.Equalf(t, "value", r2.Header.Get("X-Original"), "existing headers must be carried over")
+		require.Emptyf(t, r.Header.Values("X-Added"), "the original request must not be modified")
+	})
+
+	t.Run("clone of a request without a header is usable", func(t *testing.T) {
+		r, err := http.NewRequest(http.MethodGet, "http://example.com", nil)
+		require.NoError(t, err)
+		r.Header = nil
+
+		r2 := cloneRequest(r)
+		require.NotPanics(t, func() { r2.Header.Set("X-Added", "value") })
+		require.Nilf(t, r.Header, "the original request must not be modified")
+	})
+}