avoid error user/group already exists

sync.sh

@@ -22,8 +22,17 @@ log_error_exit() {
 if [ "$UNISON_USER" != "root" ]; then
   log_heading "Setting up non-root user ${UNISON_USER}."
   HOME="/home/${UNISON_USER}"
-  addgroup -g $UNISON_GID -S $UNISON_GROUP
-  adduser -u $UNISON_UID -D -S -G $UNISON_GROUP $UNISON_USER
+
+  if [ ! $(getent group ${UNISON_GROUP}) ]; then
+    log_info "Creating group ${UNISON_GROUP}"
+    addgroup -g $UNISON_GID -S $UNISON_GROUP
+  fi
+
+  if [ ! $(getent passwd ${UNISON_USER}) ]; then
+    log_info "Creating user ${UNISON_USER}"
+    adduser -u $UNISON_UID -D -S -G $UNISON_GROUP $UNISON_USER
+  fi
+
   mkdir -p ${HOME}/.unison
   chown -R ${UNISON_USER}:${UNISON_GROUP} ${HOME}
 fi