Please go to “Settings -> WP Related Items (WRI)” menu, chose the third tab or following tabs, according to the page types on which you would like to change the list most display to thumbnail mode (e.g chose Posts Page tab). On this tab in the “List/Thumbnail” column choose Thumbnails in every row.
I do not now the reason of value duplication. This field type does not allow choosing a value more times, I can not reproduce this problem.
Could you send us more information about it? When did this behavior begin? Have you made some changes, or could be a specific reason of this?