Greetings!

Normally, the "swapon /swapfile" command takes the default swap partition (I do not remember the size) and makes it available. This way, the kernel has the physical memory plus the swap to play with, taking your 16MB up to 16+x. What this did was to create another "swap" space, using space from the root partition, and adding it as well to give you 16+x+y MB of RAM - enough to get you through your sync.

Note - I have not seen the duplicate block problem before, so you will not need to do this for every sync going forward. I think this was more a special case, based on the type of error that fsck found.