Discussed at length in this thread: "Order by post time" rocks. "Order by relevance" sucks.
Another way to search is to use google and append freerepublic (which is how I do a search to find some of my postings from time gone by). What one has to weigh I think is the time it takes to post versus doing a real good search. I can post an article in less than 60 seconds. Wading through searches and checking can easily double that time and more (and when I am on a roll while at work time can be of the essence to me).
Perhaps a future development that could be done would be to have search built in to posting using several variables and showing you other articles which are related during preview phase. At any rate the search feature could be better but if one really wants to using the current one they could find if something has been posted (and as an aside, is there a way other than google you can search your own posts?)
I will go make coffee and perhaps I will be thinking more clearly :)