diff --git a/bin/letsencrypt-certonly.sh b/bin/letsencrypt-certonly.sh
index 7177258..3027e97 100755
--- a/bin/letsencrypt-certonly.sh
+++ b/bin/letsencrypt-certonly.sh
@@ -57,7 +57,13 @@ done
 # check for domain (hostname)
 if [[ -z $domain ]]; then
   echo "domain (hostname) is required"
-  exit
+  exit 1
+fi
+
+# check for existing pem file
+if [[ -f /etc/ssl/letsencrypt/$domain.pem ]]; then
+  echo "pem file for $domain already exists"
+  exit 1
 fi
 
 # set vars