Why do some members appear twice?
This situation normally only applies to MemberTies Professional.
The most common reason a membership record appears more than once on the Membership List is multiple addresses. When you add more than one address on a membership record, and have address fields showing as part of the Membership List view, the system will automatically use only the Active address to get the address data.
However, if you run a Search, there is a Current Address field that can be used to tell the system whether that's what you want to do. By default, the system assumes you only want the active address, but you could set it the other way, causing the member to appear once for each additional address they have. See Address Overview for details.