diff --git a/tests/update_t_client_ips.sh b/tests/update_t_client_ips.sh
index e7b58ba2..96e38260 100755
--- a/tests/update_t_client_ips.sh
+++ b/tests/update_t_client_ips.sh
@@ -3,5 +3,14 @@
 # This --up script caches the IPs handed out by the test VPN server to a file
 # for later use.
 
-echo "EXPECT_IFCONFIG4_$TESTNUM=$ifconfig_local" >> $TOP_BUILDDIR/t_client_ips.rc
-echo "EXPECT_IFCONFIG6_$TESTNUM=$ifconfig_ipv6_local" >> $TOP_BUILDDIR/t_client_ips.rc
+RC="$TOP_BUILDDIR/t_client_ips.rc"
+
+grep EXPECT_IFCONFIG4_$TESTNUM= $RC > /dev/null 2>&1
+if [ $? -ne 0 ]; then
+    echo "EXPECT_IFCONFIG4_$TESTNUM=$ifconfig_local" >> $RC
+fi
+
+grep EXPECT_IFCONFIG6_$TESTNUM= $RC > /dev/null 2>&1
+if [ $? -ne 0 ]; then
+    echo "EXPECT_IFCONFIG6_$TESTNUM=$ifconfig_ipv6_local" >> $RC
+fi