If cron entry is already present, don't duplicate it.

src/debian/postinst

@@ -4,8 +4,13 @@ set -e
 
 case "$1" in
     configure)
-        # Add the cronjob to the current user's crontab
-        (crontab -l 2>/dev/null || true; echo "0 0 * * * /usr/share/purge-backups/purge-backups-scheduler") | crontab -
+        # Check if the cronjob is already present
+        if ! crontab -l | grep -q "/usr/share/purge-backups/purge-backups-scheduler"; then
+            # Add the cronjob to the current user's crontab
+            (crontab -l 2>/dev/null || true; echo "0 0 * * * /usr/share/purge-backups/purge-backups-scheduler # Part of purge-backups package. DO NOT CHANGE!") | crontab -
+        else
+            echo "Cronjob already exists. Skipping addition."
+        fi
         ;;
     *)
         # Do nothing on other actions