check usergroup before adding

build/scripts/build_helper.upf

@@ -160,6 +160,10 @@ vpp_upf_init(){
   $SUDO cp -rf $OPENAIRCN_DIR/vpp/build-root/install-vpp-native/vpp/lib/libnat.so.21.01.1 /usr/lib/x86_64-linux-gnu/
   $SUDO ldconfig
   echo_info "Copied libraries to /usr/lib/x86_64-linux-gnu/"
-
-  $SUDO groupadd vpp
+  if [ $(getent group vpp) ]; then
+     echo "group vpp not added since it already exists"
+  else
+     $SUDO groupadd vpp
+     echo "group vpp added"
+  fi
 }