diff --git a/sgsn/SGSN_Tests.ttcn b/sgsn/SGSN_Tests.ttcn
index a7cc57f..b146046 100644
--- a/sgsn/SGSN_Tests.ttcn
+++ b/sgsn/SGSN_Tests.ttcn
@@ -1470,6 +1470,56 @@
 	vc_conn.done;
 }
 
+/* Test MS sending a duplicate Deact PDP Ctx (OS#3956). */
+private function f_TC_attach_pdp_act_deact_dup(charstring id) runs on BSSGP_ConnHdlr {
+	var PdpActPars apars := valueof(t_PdpActPars(mp_ggsn_ip));
+	var Gtp1cUnitdata g_ud;
+	var integer i;
+	var OCT1 cause_regular_deact := '24'O;
+
+	/* first perform regular attach + PDP context act */
+	f_TC_attach(id);
+	f_pdp_ctx_act(apars);
+
+	f_send_l3_gmm_llc(ts_SM_DEACT_PDP_REQ_MO(apars.tid, cause_regular_deact, false, omit), 0);
+	f_send_l3_gmm_llc(ts_SM_DEACT_PDP_REQ_MO(apars.tid, cause_regular_deact, false, omit), 0);
+
+	for (i := 0; i < 2; i := i+1) {
+		GTP.receive(tr_GTPC_MsgType(?, deletePDPContextRequest, apars.ggsn_tei_c)) -> value g_ud {
+			var integer seq_nr := oct2int(g_ud.gtpc.opt_part.sequenceNumber);
+			log("Received deletePDPContextResponse " & int2str(i) & ", seq_nr=" & int2str(seq_nr));
+			GTP.send(ts_GTPC_DeletePdpResp(g_ud.peer, seq_nr, apars.sgsn_tei_c, '7F'O));
+		}
+	}
+
+	alt {
+	[] BSSGP[0].receive(tr_SM_DEACT_PDP_ACCEPT_MT(apars.tid)) {
+		setverdict(pass);
+		}
+	[] as_xid(apars, 0);
+	}
+
+	/* Make sure second DeactPdpAccept is sent: */
+	timer T := 2.0;
+	T.start;
+	alt {
+	[] BSSGP[0].receive(tr_SM_DEACT_PDP_ACCEPT_MT(apars.tid)) {
+		setverdict(fail, "Second SM_DEACT_PDP_ACCEPT_MT received");
+		}
+	[] T.timeout {
+		setverdict(pass);
+		}
+	}
+
+	setverdict(pass);
+}
+testcase TC_attach_pdp_act_deact_dup() runs on test_CT {
+	var BSSGP_ConnHdlr vc_conn;
+	f_init();
+	vc_conn := f_start_handler(refers(f_TC_attach_pdp_act_deact_dup), testcasename(), g_gb, 46);
+	vc_conn.done;
+}
+
 /* ATTACH + ATTACH (2nd) */
 private function f_TC_attach_forget_tlli_attach(charstring id) runs on BSSGP_ConnHdlr {
 	g_pars.t_guard  := 5.0;
@@ -2545,6 +2595,7 @@
 	execute( TC_attach_pdp_act_ggsn_reject() );
 	execute( TC_attach_pdp_act_user_deact_mo() );
 	execute( TC_attach_pdp_act_user_deact_mt() );
+	execute( TC_attach_pdp_act_deact_dup() );
 	execute( TC_attach_second_attempt() );
 	execute( TC_attach_restart_ctr_echo() );
 	execute( TC_attach_restart_ctr_create() );
diff --git a/sgsn/expected-results.xml b/sgsn/expected-results.xml
index 0b477d1..d9c3706 100644
--- a/sgsn/expected-results.xml
+++ b/sgsn/expected-results.xml
@@ -47,6 +47,7 @@
   <testcase classname='SGSN_Tests' name='TC_attach_pdp_act_ggsn_reject' time='MASKED'/>
   <testcase classname='SGSN_Tests' name='TC_attach_pdp_act_user_deact_mo' time='MASKED'/>
   <testcase classname='SGSN_Tests' name='TC_attach_pdp_act_user_deact_mt' time='MASKED'/>
+  <testcase classname='SGSN_Tests' name='TC_attach_pdp_act_deact_dup' time='MASKED'/>
   <testcase classname='SGSN_Tests' name='TC_attach_second_attempt' time='MASKED'>
     <failure type='fail-verdict'>Tguard timeout
       SGSN_Tests.ttcn:MASKED SGSN_Tests control part