Hello Subash,
Short domain will matter for human because it is easy to remember and for google it is same as long domain.Having 4 character or 14 doesn't matter for google. talking about article directories the method you are going to use is blackhat. Duplicate content will not matter any, not even 0% when you are taking black path. If it was white hat then we shouldn't have thought about duplicate content.
Never worry and move ahead. Since you are using wordpress there are plugins which make all of your articles copyscape passed. If you don't install plugin also there won't be any problem. Duplicate content is just a rumor only. However, i will suggest to use different title so that you can easily rank on google.com
I have made near about 25 sites for my clients which are only full of duplicate articles and contents.Few of them are ranked higher than the original and most of them just below the original.
Wish you all the best for your new site. The articles directories are best way, once again i say - the best way ever to be successful without investment and hard effort. (However note that it is black-hat method)