diff --git a/gather.go b/gather.go
index 73359de8..d5315d7a 100644
--- a/gather.go
+++ b/gather.go
@@ -255,6 +255,7 @@ func (a *Agent) gatherCandidatesLocalUDPMux(ctx context.Context) error { //nolin
 	}
 
 	localAddresses := a.udpMux.GetListenAddresses()
+	configs := make(map[CandidateHostConfig]struct{})
 
 	for _, addr := range localAddresses {
 		udpAddr, ok := addr.(*net.UDPAddr)
@@ -272,10 +273,6 @@ func (a *Agent) gatherCandidatesLocalUDPMux(ctx context.Context) error { //nolin
 			candidateIP = mappedIP
 		}
 
-		conn, err := a.udpMux.GetConn(a.localUfrag, udpAddr)
-		if err != nil {
-			return err
-		}
 		hostConfig := CandidateHostConfig{
 			Network:   udp,
 			Address:   candidateIP.String(),
@@ -283,6 +280,20 @@ func (a *Agent) gatherCandidatesLocalUDPMux(ctx context.Context) error { //nolin
 			Component: ComponentRTP,
 		}
 
+		// Detect a duplicate candidate before calling addCandidate().
+		// otherwise, addCandidate() detects the duplicate candidate
+		// and close its connection, invalidating all candidates
+		// that share the same connection.
+		if _, ok := configs[hostConfig]; ok {
+			continue
+		}
+		configs[hostConfig] = struct{}{}
+
+		conn, err := a.udpMux.GetConn(a.localUfrag, udpAddr)
+		if err != nil {
+			return err
+		}
+
 		c, err := NewCandidateHost(&hostConfig)
 		if err != nil {
 			closeConnAndLog(conn, a.log, fmt.Sprintf("Failed to create host mux candidate: %s %d: %v", candidateIP, udpAddr.Port, err))
diff --git a/gather_test.go b/gather_test.go
index 448a34ed..8cbef5d7 100644
--- a/gather_test.go
+++ b/gather_test.go
@@ -599,6 +599,51 @@ func TestTURNProxyDialer(t *testing.T) {
 	assert.NoError(t, a.Close())
 }
 
+// TestUDPMuxDefaultWithNAT1To1IPsUsage asserts that candidates
+// are given and connections are valid when using UDPMuxDefault and NAT1To1IPs.
+func TestUDPMuxDefaultWithNAT1To1IPsUsage(t *testing.T) {
+	report := test.CheckRoutines(t)
+	defer report()
+
+	lim := test.TimeOut(time.Second * 30)
+	defer lim.Stop()
+
+	conn, err := net.ListenPacket("udp4", ":0")
+	assert.NoError(t, err)
+	defer func() {
+		_ = conn.Close()
+	}()
+
+	mux := NewUDPMuxDefault(UDPMuxParams{
+		UDPConn: conn,
+	})
+	defer func() {
+		_ = mux.Close()
+	}()
+
+	a, err := NewAgent(&AgentConfig{
+		NAT1To1IPs:             []string{"1.2.3.4"},
+		NAT1To1IPCandidateType: CandidateTypeHost,
+		UDPMux:                 mux,
+	})
+	assert.NoError(t, err)
+
+	gatherCandidateDone := make(chan struct{})
+	assert.NoError(t, a.OnCandidate(func(c Candidate) {
+		if c == nil {
+			close(gatherCandidateDone)
+		} else {
+			assert.Equal(t, "1.2.3.4", c.Address())
+		}
+	}))
+	assert.NoError(t, a.GatherCandidates())
+	<-gatherCandidateDone
+
+	assert.NotEqual(t, 0, len(mux.connsIPv4))
+
+	assert.NoError(t, a.Close())
+}
+
 // Assert that candidates are given for each mux in a MultiUDPMux
 func TestMultiUDPMuxUsage(t *testing.T) {
 	report := test.CheckRoutines(t)