Freezing isn't the problem, as much as thawing, and dehydrating. As long as you don't allow bacteria in when you are thawing to ruin the skin, you can freeze as many times as it takes. Also, don't re-freeze and allow too much air in the bag that you freeze your skin in, or you will dehydrate the skin, and it will not remain soft and plyable. What I do is this. I skin the birds, maybe 10 at a time. I take all my body and neck measurements for my order (try Tony Finazzo or Frank Newmeyer, they carry a large selection) I vacuum wrap them in a Food Saver, and re-freeze them. The next time I get them out, I thaw them in cold water or at room temperature. I de-fat them, I wash them, and re-freeze them in their original (cleaned) bag. Leave them wet, and vacuum wrap them again. You will have to put a piece of paper towel at the opening of the bag to suck up excess water to make a proper seal. Once you have done this, these skins will last a very long time. Basically they are frozen in a block of ice, and take up very little space in the freezer. I wouldn't think it would hurt to freeze and thaw your duck/skin for every step of the skinning and fleshing process. Just don't let them get too warm during these steps.