Official Google Webmaster Central Blog: Demystifying the “duplicate content penalty”
That’s not to say that there isn’t a benefit to creating unique content for each of your posts. Google employs a duplicate content filter that limits how many duplicates will appear on the SERP for a given keyword query. By creating unique posts on each forum you increase the odds of having more than one of your posts appear in a single search query.
Now, on to the automation question, this really gets into the policies and Terms of Service of each of the forums where you are posting. They all must devote much of their time and resources to fighting spam. And due to the relentless onslaught of spam attacks on the typical forum, they are likely to have strict policies, which might include barring automated posts. You should check the policies of each forum to see what is permitted.