Friedman Index of Coincidence, pruning multiples of the keylength
I suspect, when the IC was invented, it was typically used together with human judgement, so there might not be any standard rules to address this issue.

Anyway, it seems like there are two plausible solutions:


It might be easy to add a special case for this situation: choose the keylength with the highest IC, except that if it has a divisor whose IC is within 10% of it, choose the divisor.
Another plausible solution would be to try running the rest of your attack on both candidates.


I would imagine either of these might work fine.