Is there a good way to avoid this? I'm still learning SVN, but what I have thought was right was to do "Update to HEAD" before doing a commit, and then making sure to take care of the conflicting files (and cards.txt always conflicts). Do you have a better process?
This is exactly how it should be done (in the Synchronize With Repository view, "Edit conflicts" - in the text compare, just make sure the left local side is correct - and hit "Mark as merged" when done). What does apparently happen though is that sometimes when a lot of changes are done on cards.txt earlier in the file, they can easily get missed. It's happened to me numerous times already, but from now on, I make sure I haven't missed a single "red difference spot" in the text compare.
I don't agree with making only one of us "master of cards.txt", because there's just too many changes that have to be done on it, all of the time. It's possible to do right, we just have to be careful.
And it's interesting, cards.txt is actually the easiest file to merge conflicts (it doesn't have Eclipse insert the weird "<<<<mine>>>>>" text parts).