Blisters on blisters!

Bought my latest pair of running shoes in April, and they're brilliant apart from one thing: I get a really bad blister every time I wear them under the arch of my right foot. I've tried a variety of different socks, and the shoes are well broken in, but they just won't stop rubbing in this one place. My previous shoes did it too, but they did stop eventually.

I'm not sure whether I'm doing permanent damage to my foot by allowing blisters to develop on top of existing blisters - even if I'm not, it's not very pretty to look at and is slightly painful by the end of a run. The skin underneath has gone hard, but the blisters still keep forming, either wearing through the hard skin or right beside it. What can I do? I'm not in the market for a new pair of shoes, so any suggestions other than that most welcome!