There are two correct answers that are available for this question. The first thing that you have to do is to select the update and type add only. After that, you can ensure that the data extension contains a primary key to be sure that only the new data will be added during the import process.
You can already imagine what will happen if all of the data will be imported. This would result in some data becoming mixed. Some may not be overwritten, and it will be more challenging to remove the duplicated data. It will also make the data harder to analyze and understand.