I did one today, but I cannot blame it on Free Republic...that was was all of my doing by accident.
However, when Free Republic is responsible, I get the distinct impression it is due to latency in the Internet. IIRC, when it happens to me, when you click the post button nothing happens...even if you wait, so you click it again...duplicate post.
At least that was how I saw it presented. Other people might see it happen differently.
Duplicates are often locked with the moderator’s message - Search Still Works.
Trouble is they don’t tell you HOW to search. Protip: DO NOT use Free Republic search. That doesn’t work.