Here is a relevant 2008 comment from Dann Corbit.
I am not sure whether Corbit would like his comment here to applied to the Rybka situation, but to me it seems valid.Dann Corbit wrote:You cannot undo authorship. Fabian will be one of the authors of Toga for all eternity whether someone erases his name or not.
Eventually, Toga may be totally dissimilar from Fruit. But the origin does not change.
Of course, one can enumerate some historical examples, where supposed discrepancies can arise due to (e.g.) indecisive partnerships in the early days of development. For instance, Nona is listed as being by Morsch in 1984 (ICCA Journal 7/4 concerning the Netherlands Computer-Chess Championship), and the "amateur" part of the 5th WMCCC (ICCA Journal 8/3) lists this again, but then for the Dutch Computer-Chess Championship in 1985 (ICCA Journal 8/4), one finds that E.G.H. Schroeder is listed as a co-author (note that a few months earlier, Rebel competed in the amateur part of the 5th WMCCC separately from Nona). Indeed, the accompanying text in the article speaks of their co-work. This appears to be the only joining of these two, as both Nona and Rebel competed separately in the WCCC and the DCCC in 1986. When counting book authors as "co-authors" (rather than co-entrants, I guess) it can be even more hairy.