diff --git a/lib/core_lib.sh b/lib/core_lib.sh
index 243c67f..d5af1e6 100755
--- a/lib/core_lib.sh
+++ b/lib/core_lib.sh
@@ -16,7 +16,12 @@
 # limitations under the License.
 #######################################################################
 
-_COMPATIBILITY_MODE=true
+# this script need only be sourced once
+if [ "$_COMPATIBILITY_MODE" = true ]; then
+	return
+else
+	_COMPATIBILITY_MODE=true
+fi
 
 #######################################################################
 # Specify the absolute path to each shell command file.