No, it does not duplicate posts in cron job mode. This is absolutely impossible in case if your settings are correct and the actual posts in the feed have the same GUID’s and titles every time you pull the feed. If titles and/or GUID’s are not permanent, course there will be duplicates. Right now CyberSEO (Lite and Pro) is being used on 68,579 websites and nobody has a duplicate issue.
So, there is something wrong with your settings or with the feed. Unfortunately I have no time to analyze your feed, check your settings etc. This service is available to customers of Pro version only.