You will need to differentiate between sources with the same author and publication date by adding a lowercase letter to the publication year. Including this lowercase letter ensures that the reader knows which source you are citing in-text and can find that source in your reference list.
Add the lowercase letter immediately after the publication year in reference entries:
Centers for Disease Control and Prevention. (2011a). Birth defects. Retrieved April 26, 2011, from http://www.cdc.gov/ncbddd/birthdefects/index.html
Centers for Disease Control and Prevention. (2011b). Calcium and bone health. Retrieved April 26, 2011, from http://www.cdc.gov/nutrition/everyone/basics/vitamins/calcium.html
Centers for Disease Control and Prevention. (2011c). Prevent diabetes. Retrieved April 26, 2011, from http://www.cdc.gov/diabetes/consumer/prevent.htm
*Note that we have listed these entries alphabetically by title, with the lowercase letters assigned based on this order. Also note that when you cite these sources in the text, include the letter with the date so that readers can match that citation to the appropriate reference entry.