Retaining all files in a group is too complicated: you need to type the whole string "all". Just the letter "a" is enough.
Created attachment 177947 [details, diff] patch to implement the enhancement This implements the enhancement in a patch to one line of source code
Thank you for report. But this was upstream decision and I don't want to change program behavior it in Gentoo only. Please report this patch/feature upstream and if he takes it, we'll have this enhancement in next fdupes release.