These are distinguished by adding lower case letters (a,b,c, etc.) after the year and within the brackets:
Then the year and lower case letter remain noted together (e.g. 2015a) in the corresponding reference you add to the list at the end of your work.
If you have citations in the main text of your work with an identical author's name and year, the added lower case letters after the year determines ordering in your reference list at the end of your work e.g. a reference starting with Johnson, C., 2015a. would be listed before Johnson, C., 2015b.