A duplicate person simply means that they appear more than once in a family tree because they are related in multiple ways. This is the standard way to handle multiple relationships in a computer program that generates family trees. If you were drawing the family tree by hand, you might draw lines connecting the people across generations or branches of the family. There is no easy way for a computer program to know how to do this to best convey the relationships.
There does seem to be a problem with how Family Echo shows multiple relationships when siblings are partners. Family Echo does what it always does, and it looks odd and overly complicated in this instance. I believe Family Echo is aware of this.
