diff --git a/go.mod b/go.mod
index 29ccbfa..d1e67bf 100644
--- a/go.mod
+++ b/go.mod
@@ -2,12 +2,14 @@ module github.com/hits/ExploreDNS
 
 go 1.25.6
 
-require github.com/miekg/dns v1.1.72
+require (
+	github.com/miekg/dns v1.1.72
+	golang.org/x/sync v0.19.0
+)
 
 require (
 	golang.org/x/mod v0.31.0 // indirect
 	golang.org/x/net v0.48.0 // indirect
-	golang.org/x/sync v0.19.0 // indirect
 	golang.org/x/sys v0.39.0 // indirect
 	golang.org/x/tools v0.40.0 // indirect
 )
diff --git a/internal/fingerprint/fingerprint.go b/internal/fingerprint/fingerprint.go
index ad1f890..d467822 100644
--- a/internal/fingerprint/fingerprint.go
+++ b/internal/fingerprint/fingerprint.go
@@ -7,6 +7,7 @@ import (
 	"time"
 
 	miekgdns "github.com/miekg/dns"
+	"golang.org/x/sync/singleflight"
 )
 
 const defaultTimeout = 2 * time.Second
@@ -17,6 +18,7 @@ type Fingerprinter struct {
 	mu       sync.Mutex
 	cache    map[string]string
 	timeout  time.Duration
+	group    singleflight.Group
 	exchange func(ctx context.Context, addr string, m *miekgdns.Msg) (*miekgdns.Msg, error)
 }
 
@@ -36,6 +38,8 @@ func NewWithTimeout(timeout time.Duration) *Fingerprinter {
 // Query returns the version string for ip, or "" if the server doesn't
 // respond or doesn't support the version.bind CHAOS query.
 // Results are cached: subsequent calls for the same IP return immediately.
+// Concurrent calls for the same IP are coalesced via singleflight so only
+// one network probe is issued per IP at a time.
 func (f *Fingerprinter) Query(ctx context.Context, ip net.IP) string {
 	key := ip.String()
 
@@ -46,13 +50,15 @@ func (f *Fingerprinter) Query(ctx context.Context, ip net.IP) string {
 	}
 	f.mu.Unlock()
 
-	version := f.probe(ctx, ip)
+	v, _, _ := f.group.Do(key, func() (interface{}, error) {
+		version := f.probe(ctx, ip)
+		f.mu.Lock()
+		f.cache[key] = version
+		f.mu.Unlock()
+		return version, nil
+	})
 
-	f.mu.Lock()
-	f.cache[key] = version
-	f.mu.Unlock()
-
-	return version
+	return v.(string)
 }
 
 // FingerprintAll queries all ips concurrently and returns a map of
diff --git a/internal/output/formatter.go b/internal/output/formatter.go
index 5cd44b7..85d8532 100644
--- a/internal/output/formatter.go
+++ b/internal/output/formatter.go
@@ -31,9 +31,6 @@ type Config struct {
 	Color              bool
 	Debug              int
 
-	// Fingerprints maps server IP strings to their version.bind version strings.
-	// Populated by RunTraversal when ShowVersions and ShowServers are both true.
-	Fingerprints map[string]string
 }
 
 func DefaultConfig() *Config {
@@ -56,6 +53,9 @@ type Formatter interface {
 	WriteResult(result traverse.TraversalResult) error
 	WriteSummary(results []traverse.TraversalResult) error
 	Flush() error
+	// SetFingerprints supplies server-version data to the formatter.
+	// Call before WriteSummary when ShowVersions is true.
+	SetFingerprints(fps map[string]string)
 }
 
 func NewFormatter(cfg *Config, w io.Writer) Formatter {
diff --git a/internal/output/formatter_test.go b/internal/output/formatter_test.go
index 1f5e774..48348bc 100644
--- a/internal/output/formatter_test.go
+++ b/internal/output/formatter_test.go
@@ -150,6 +150,80 @@ func TestJSONFormatterProducesValidOutput(t *testing.T) {
 	}
 }
 
+func TestTextFormatterWriteSummaryShowsVersions(t *testing.T) {
+	ref := traverse.NewReferral("example.com.", dns.TypeA, "com.", 0, 1.0, nil)
+	serverIP := net.ParseIP("198.41.0.4")
+	resp := &traverse.Response{
+		Referral: ref,
+		Server:   serverIP,
+		Type:     traverse.RespAnswer,
+	}
+
+	var buf bytes.Buffer
+	cfg := DefaultConfig()
+	cfg.Color = false
+	cfg.ShowVersions = true
+	cfg.ShowServers = true
+	cfg.ShowResults = false
+	cfg.ShowSummaryResults = false
+	formatter := NewFormatter(cfg, &buf)
+	formatter.SetFingerprints(map[string]string{serverIP.String(): "BIND 9.18.1"})
+
+	if err := formatter.WriteSummary([]traverse.TraversalResult{{Referral: ref, Response: resp}}); err != nil {
+		t.Fatalf("WriteSummary: %v", err)
+	}
+
+	out := buf.String()
+	if !strings.Contains(out, "BIND 9.18.1") {
+		t.Fatalf("expected version string in text output, got %q", out)
+	}
+}
+
+func TestJSONFormatterWriteSummaryShowsVersions(t *testing.T) {
+	ref := traverse.NewReferral("example.com.", dns.TypeA, "com.", 0, 1.0, nil)
+	serverIP := net.ParseIP("198.41.0.4")
+	resp := &traverse.Response{
+		Referral: ref,
+		Server:   serverIP,
+		Type:     traverse.RespAnswer,
+	}
+
+	var buf bytes.Buffer
+	cfg := DefaultConfig()
+	cfg.Format = FormatJSON
+	cfg.Domain = "example.com"
+	cfg.QueryType = "A"
+	cfg.ShowVersions = true
+	cfg.ShowServers = true
+	cfg.ShowResults = false
+	cfg.ShowSummaryResults = false
+	formatter := NewFormatter(cfg, &buf)
+	formatter.SetFingerprints(map[string]string{serverIP.String(): "Unbound 1.17.0"})
+
+	if err := formatter.WriteSummary([]traverse.TraversalResult{{Referral: ref, Response: resp}}); err != nil {
+		t.Fatalf("WriteSummary: %v", err)
+	}
+	if err := formatter.Flush(); err != nil {
+		t.Fatalf("Flush: %v", err)
+	}
+
+	var payload map[string]any
+	if err := json.Unmarshal(buf.Bytes(), &payload); err != nil {
+		t.Fatalf("invalid json: %v\n%s", err, buf.String())
+	}
+	servers, ok := payload["servers"].([]any)
+	if !ok || len(servers) == 0 {
+		t.Fatalf("expected servers in json output, got %v", payload)
+	}
+	srv, ok := servers[0].(map[string]any)
+	if !ok {
+		t.Fatalf("expected server object, got %T", servers[0])
+	}
+	if srv["version"] != "Unbound 1.17.0" {
+		t.Fatalf("expected version = %q, got %v", "Unbound 1.17.0", srv["version"])
+	}
+}
+
 func TestRunTraversalUsesHooks(t *testing.T) {
 	answerResp := func() *miekgdns.Msg {
 		m := new(miekgdns.Msg)
diff --git a/internal/output/json.go b/internal/output/json.go
index 29c4200..407b6c8 100644
--- a/internal/output/json.go
+++ b/internal/output/json.go
@@ -9,9 +9,10 @@ import (
 )
 
 type jsonFormatter struct {
-	cfg     *Config
-	w       io.Writer
-	payload jsonDocument
+	cfg          *Config
+	w            io.Writer
+	payload      jsonDocument
+	fingerprints map[string]string
 }
 
 type jsonDocument struct {
@@ -71,6 +72,10 @@ func newJSONFormatter(cfg *Config, w io.Writer) *jsonFormatter {
 	}
 }
 
+func (f *jsonFormatter) SetFingerprints(fps map[string]string) {
+	f.fingerprints = fps
+}
+
 func (f *jsonFormatter) WriteProgress(event traverse.TraversalEvent) error {
 	if !f.cfg.ShowProgress {
 		return nil
@@ -106,9 +111,9 @@ func (f *jsonFormatter) WriteSummary(results []traverse.TraversalResult) error {
 		servers := collectServers(results)
 		for name, ips := range servers {
 			srv := jsonServer{Name: name, IPs: ips}
-			if f.cfg.ShowVersions && f.cfg.Fingerprints != nil {
+			if f.cfg.ShowVersions && f.fingerprints != nil {
 				for _, ip := range ips {
-					if v := f.cfg.Fingerprints[ip]; v != "" {
+					if v := f.fingerprints[ip]; v != "" {
 						srv.Version = v
 						break
 					}
diff --git a/internal/output/runner.go b/internal/output/runner.go
index 67c6b6b..4e3139b 100644
--- a/internal/output/runner.go
+++ b/internal/output/runner.go
@@ -31,7 +31,8 @@ func RunTraversal(ctx context.Context, traverser *traverse.Traverser, cfg *Confi
 	// Gating on ShowServers avoids unnecessary network calls when versions
 	// would not be displayed anyway.
 	if cfg.ShowVersions && cfg.ShowServers {
-		cfg.Fingerprints = fingerprint.New().FingerprintAll(ctx, collectUniqueServerIPs(results))
+		fps := fingerprint.New().FingerprintAll(ctx, collectUniqueServerIPs(results))
+		formatter.SetFingerprints(fps)
 	}
 
 	if err := formatter.WriteSummary(results); err != nil {
diff --git a/internal/output/text.go b/internal/output/text.go
index a5f6fae..5933b62 100644
--- a/internal/output/text.go
+++ b/internal/output/text.go
@@ -11,14 +11,19 @@ import (
 )
 
 type textFormatter struct {
-	cfg *Config
-	w   io.Writer
+	cfg          *Config
+	w            io.Writer
+	fingerprints map[string]string
 }
 
 func newTextFormatter(cfg *Config, w io.Writer) *textFormatter {
 	return &textFormatter{cfg: cfg, w: w}
 }
 
+func (f *textFormatter) SetFingerprints(fps map[string]string) {
+	f.fingerprints = fps
+}
+
 func (f *textFormatter) WriteProgress(event traverse.TraversalEvent) error {
 	if event.Stage != traverse.EventStart {
 		return nil
@@ -101,7 +106,7 @@ func (f *textFormatter) writeServers(results []traverse.TraversalResult) error {
 		for _, ip := range servers[name] {
 			line := fmt.Sprintf("%*s: %-15s", width, name, ip)
 			if f.cfg.ShowVersions {
-				if version, ok := f.cfg.Fingerprints[ip]; ok && version != "" {
+				if version, ok := f.fingerprints[ip]; ok && version != "" {
 					line += "  " + version
 				}
 			}