In every political list ever seen, there are duplicate email addresses. Most often, these represent couples who use the same email. Nationbuilder uses email addresses as unique identifiers and require importers to adjust data before import. They suggest adjusting the data to remove duplicate email addresses. This is terrible practice but acceptable for small campaigns. For political campaigns, it is not acceptable. Nationbuilder does not even allow the population of a second email via data import. All in all, this is a major weakness in the product.