diff --git a/functions b/functions
index 97167b4..f472389 100644
--- a/functions
+++ b/functions
@@ -196,6 +196,12 @@ iface_is_up()
 	$IP -o link show dev $NAME 2>/dev/null | cut -d' ' -f3 | grep -qs '[<,]UP[,>]'
 }
 
+iface_have_ipaddr()
+{
+	local NAME=${1:?missing 1st argument to $FUNCNAME}
+	$IP -o addr show dev $NAME 2>/dev/null | egrep -qs " $NAME +inet ([0-9]{1,3}\.){3}[0-9]{1,3}/[0-9]{1,2} "
+}
+
 # Invoke program which understands the "-o NAME" option -
 # insmod for modutils, modprobe for module-init-tools.
 modprobe_with_rename()
@@ -494,7 +500,7 @@ eval_string()
 
 declare -frx is_yes is_no SourceIfNotEmpty ExecIfExecutable supported_type name2type
 declare -frx pickup_options print_error print_message print_progress print_nack
-declare -frx iface_is_up try_rmmod get_mapped_ifname profiled_filename init_netprofile
+declare -frx iface_is_up iface_have_ipaddr try_rmmod get_mapped_ifname profiled_filename init_netprofile
 declare -frx init_nethost foreach_child ifdown_children ifup_children ifup_parents
 declare -frx ifdown_parents iface_exists seen_iface add_seen_iface xargise_file
 declare -frx have_ifplugd pickup_defaults trim need_detection stop_dhcp_client
diff --git a/ifup-removable b/ifup-removable
index 0bcb556..e788ae3 100755
--- a/ifup-removable
+++ b/ifup-removable
@@ -52,6 +52,10 @@ export IFACEDIR MYIFACEDIR SCRIPTDIR NAME NETPROFILE SEEN_IFACES
 init_netprofile
 if [ ! -d "$MYIFACEDIR" ]; then
 	if is_yes "$ALLOW_UNKNOWN"; then
+		if iface_is_up $NAME && iface_have_ip $NAME; then
+			print_message -n "unknown interface '$NAME' already configured - skipping (lookup ALLOW_UNKNOWN_RESET option or create a usable configuration)"
+			exit 1
+		fi
 		if [ -d $IFACEDIR/unknown@$NETHOST ]; then
 			MYIFACEDIR=$IFACEDIR/unknown@$NETHOST
 		else