Publishing again acknowledging the original publication of oneself I have come across the International Journal of Advanced Computer Science and Applications(IJACSA).  The copyright section says that "Authors retain the right to publish their material elsewhere, providing the original publication is acknowledged."

Can the same paper be sent to somewhere else as well. Given that I have seen that mostly conferences/journals require unpublished material, what does this clause exactly mean ? Is it normal clause in Open Access Journals ?
That means that IJACSA will not try to stop you from publishing your work elsewhere.  (They'd have little reason to do so, as they are not profiting from it.)  It is common for open-access journals to let the author retain the publication rights.

However, no reputable journal will publish an article that has already appeared somewhere else.

You could "publish" it on your web page, or on arXiv, or as part of a book (though again, a reputable book publisher will probably balk if the content is just recopied).

Note: IJACSA is published by The Science and Information Organization, which is on Beall's List of Predatory Open-Access Publishers.  Its website makes the journal look very dubious, especially the ridiculously broad scope and the promise to complete peer review within 15 days.  I wouldn't publish there (or would I?).