When cousins marry (in the 1800s before you judge me!), the tree doesn't connect the two in part of a 'circle' or loop, (formed by the cousins/spouses, their parents and their grandparents), which would display the information most clearly. Instead, it shows the cousins twice, but with the word 'Duplicate' on their squares.
This makes for confusing reading and surely would not be difficult to fix. This problem gets quite prevelent as the tree goes back more than a few hundred years, so please could it be looked into? A loop would display the information much more clearly.