I haven’t (re-?)read Fodor’s stuff yet, but if I followed correctly, Fodor's general point was that there arguably are pairs of distinct properties such that, nonetheless, there is no fact of the matter as to whether a given mental state has the function of signalling one as opposed to the other. (As Oscar remarked, plausible examples might be harder to come with if a restriction to natural (enough) properties is in place.)
This sounds right. But, as Sebas also worried, it’s not clear in which sense the resulting indeterminacy is not precisely one the defender of the teleological/functional proposal would independently predict and willingly embrace.