If you wish to upload any voter file other than your first, you need to be aware of how Merge works, to ensure that any conflicts that may exist between the data sets are correctly resolved.
Ecanvasser offers two methods of Merge.
If during your initial File Import, you selected to include a Contact ID; then you may use this as a unique key to match contacts during any subsequent import, assuming that the new files contain the same matching Contact ID.
You can learn more about this here.
Name & Address
Particularly outside of the United States, many countries do not have a reliable Contact ID system that can be used for merges. In such instances, Ecanvasser can parse your subsequent voter files, and identify any potential conflicts. These conflicts can then be reviewed by a member of your team for resolution.
You can learn more about how this works here.