If there are duplicate values in your deal page drop-down, it is due to the way that custom attributes have been added to the products.
For example, if one product in a group has the Color listed as "Blue",
while another product in the group has the Color listed as "Blue" with the Custom Color value "Blue",
they will be read as two different values (Blue vs. Blue//Blue) and duplicated in the drop-down menu.
To fix this issue, delete the custom value from the Color attribute so that all attributes for that color match.
Another reason for this may be because one custom color may have a space after it while another option with the same color doesnt have a space after it. Please delete the additional space and save.
NOTE: The attributes and custom attributes will appear the same in the attributes next to the products on the Update Product Group page. You will need to click into the products to see if the attributes have custom color.