This is one of the major confusing problems with "Family View" graphic display.

The program cannot draw zig-zag lines between marrying cousins, to show that each has unique ancestry in the display. Instead, it shows X with spouse Y in one place, and Y with spouse X in another place on the screen, and you can open up each one's ancestry display in each place to make the program display what looks like a whole lot of duplicates.

As others have suggested, the place to look to ensure you do not have duplicated entries for one person is in your tree's List of Persons.