Hi OP,
A duplicate in the family tree essentially means that an individual has multiple relations to the selected person.
For instance, If my mother's sister is also my dad's brother's wife, she will show up twice.
I'm assuming it's something similar in your case as well, with one person having atleast 2 relations with the person you've currently selected.
When you say you'd like to hide duplicates, what do you exactly mean? Taking my example from above, what would hiding duplicates even look like?
Curious to hear from you, hope my answer helped.