As you are probably aware, duplicate people appear in your tree when two people are related in multiple ways. FamilyEcho has decided that instead of having multiple lines connecting individuals to show these relationships, The individuals in question will have an additional box in the family tree in the appropriate spot.
These are marked as "duplicates" to show that there are multiple relationships. But there is only one record for each individual, and editing either box will change the information for that person.
The other way to show multiple relationships is to have multiple lines between people. This can look fine when there are just a few to show in a tree, but can get very complicated and maybe confusing when you have more or when they cross over between generations. Because there was no simple solution to having a legible family tree in all cases without duplicates, FamilyEcho decided to use duplicates to show multiple relationships. There is no option to change this.