I think I'have found core funcionality bug in Magento 22.214.171.124 (rwd). Here is how all things must be done:
1) Register some user
2) Add billing and shipping address and few ( two or three) additional addresses
3) Make sure you are enabled multiple addresses checkout
4) Add few products to cart
5) Go to checkout page to multiple addresses step, at index.php/checkout/multishipping/addresses/
6) Select DIFFERENT addresses to each products and click continue
7) After you are redirected to shipping information page just go directly to your catalog, select some product and add it to cart and look then what subtotal price in cart is calculated It is multiplied depending on how much different addresses you have selected at multishipping/addresses page.
Seems that app/code/core/Mage/Sales/Model/Quote.php -> collectTotals() method is calculating wrong subtotal prices in this case or maybe there are some other issues.. If anyone else had this issue and know how to fix this please contact me.
Thank you for your report.
Have you checked the bug tracking system that there is no other that has reported the same problem?
The system for bug repporting and checking for simular cases you find here.