Use a column of unique data.
Another way to make data easier to work with in Visio is to make sure that a column in the data source contains unique values. When you refresh the data linked to a diagram, Visio needs to identify which row in the External Data window (and thus which shape in the diagram) to apply the updates to. A column with unique values, such as names or ID numbers, will ensure this happens smoothly.
In the picture above, example A demonstrates this. In example B, there are duplicate values in the ID column. That could cause confusion for Visio and also for you —how will you tell which row is which?
Note A column of unique values is also known as a key column or primary key. If you're using a database as your data source, this is most likely all ready for you. But if you're using Excel, you should ensure that such a column exists.