EDIT: Just played with my friend. The rule was that whoever thought of a repeat word first, lost.
would be fun but near-impossible to do in a long-term forum game format.
@natori @IncrediblePal and I at times do search within the thread in some of the other word games to see if a word was used before, or at least how recently. it's less of a pain in this game than those like "change only one letter" since the number of associations is orders of magnitude greater than new words made by only changing one letter, for example.