"*Note: Skip to the bottom of the review if you just want the weights of the packs (I only have nine).
These are nice additions to the Mario LEGO theme sets. It's a bit disappointing that you don't know which character you are getting until after you purchase; that's going to make it difficult to collect all ten characters. However, there is something you might be able to do to help prevent duplicates: weigh the packs.
I purchased all the Mario sets when they first came out directly from lego.com, including three character packs. Fortunately, the three characters were all different. Instead of hoping against luck that I fill in the rest of the collection through random purchases, I purchased a pocket scale with a sensitivity of 0.01 grams ($17, but you can find one for around $10) in the hopes of being able to prevent duplicate purchases.
I visited a LEGO store in person. They had about thirty to forty character packs. I spent about about thirty minutes in the store weighing all the packs they had. I already knew the weights of the three packs I had purchased, so I was looking for seven additional packs all having different weights. I think the effort was very helpful. There will be variances between the weights of the bags and possibly the weights of the pieces, and there could be the possibility (I'm not sure) that different packs of the same character type have a different amount of extra pieces. I was trying to take these nuances into account, and I was arbitrarily choosing a difference of less than 0.1 grams as the same characters.
I was confident about six of the packs (there was a difference of at least 0.17 grams between any two packs). I wasn't sure about the last one, so I purchased two that were close to the weight of others. It turns out those two extra were, in fact, duplicates. I opened one, but the other one I could feel (which I did once I was home after I purchased) was a duplicate. So, either the store did not have the last one I'm missing, or the weight of the last one is too close to one of the other packs (which will make getting it difficult).
Here is the list of characters packs with their total weights (including bag) and weights of the empty bags, respectively. I have nine unique characters. I have listed one duplicate with bag and one duplicate without bag (since I didn't open my second duplicate). I will update with the last one once I get Urchin.