Combining Multiple Responses from Single User

Hello,
I have a survey that will be sent out monthly for a certain period of time to the same distribution email list. Depending on where they are in the process, the recipients will answer different questions each time they receive the survey. For example, the first time they receive the survey, a recipient may answer questions 1 and 2. The next time he/she may answer questions 3 and 4. In this case I will have two records of responses for the same recipient. How do I combine a recipient's multiple responses so that I end up with one row per recipient with all of their answers?