Help with Countif and Or

Hello! 

I'm trying to get a total number of people who have completed surveys. Some people complete their surveys after 3 months, the date that they complete their survey is added to a 3 month survey column. Some people complete it after 6 months, and the date is added to a 6 month survey column. Sometimes there will be a date in both columns but I don't want those people counted twice. What formula could I use?