MLA multiple parenthetical citation essay without page numbers
Well, normally with MLA in-text citations, you should have the author's name in the parenthetical citations followed by the page number. Since you do not have a page number, you could remedy this by putting the chapter name in quotes instead ( e.g. (Levitt, "The Hidden Side of Everything")). If neither of these are available, you could put the name of the piece in the citations in quotes since you have already said the author's name in the sentence (e.g. ("Freakonomics")).

MLA Rules: https://owl.english.purdue.edu/owl/resource/747/2/
Chapter Name in Citation: http://www.easybib.com/guides/citation-guides/mla-format/how-to-cite-a-parenthetical-citations-mla/