I dunno. Consider a progressive income tax/social security system as applied only to the retired population. There are certainly cross-transfers occurring among the elderly. But I can’t see where the efficiency loss comes in.
And as Eris points out, there’s no particular reason to believe that the initial status quo was an efficient allocation.
Furthermore, there’s nothing wrong to preferring a more equitable outcome to a less one. It’s just that preferences for societal equity cover a wide range, and there are no stemwinder arguments on the conventional left or right for any particular distribution of wealth or income (save perhaps those of Pareto and Rawls - but few are calling for a Soviet or Maoist solution anymore).