A few customers have complained that they are getting subtotal and grand total prices that are double of what they should be. After some tests, I've found that when a user adds a product to cart, go to checkout, then changes his mind and removes the item from the cart - the next time he will add a product to the cart, the subtotal and grand total prices will get doubled. It happened only in IE and Firefox, not in Chrome.
I've checked the DB table sales_flat_quote_address and saw there were quote ids that had more than one shipping and billing address.
I implemented the solution that is shown in this post: https://web.archive.org/web/20150227035906/http://www.magentocommerce.com/boards%20/viewthread/227560/P15/#t393997
and the price is no longer getting doubled in the situation I described above.
The problem is that since this fix I still get occasionally a complaint from customers about the price being doubled and I see the same situation in the DB table sales_flat_quote_address, but I can't reproduce this issue so I don't know when it happens.
Any ideas what I can do?