Not sure how old this question is, but I noticed the only response didn't answer your question at all. There are two possible reasons to get blisters only on one foot:
1) Most likely, it has to do with the way you run or walk. Most of us have a dominant leg/foot, just as we have a dominant arm/hand, and there is a natural tendency to stay slightly more centered over your dominant foot, assigning it more load and control. Depending on how much we favor one foot over the other when we walk or run, it causes significant differences in the way each foot interacts with the ground and with its shoe.
2) A less likely reason could be your shoes. Even machine-made shoes have imperfections all over them, including on the insoles. If you get the same blisters no matter what shoes you're wearing, however, then this is probably not your problem.
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