I've asked this on other sites but nobody seems to remember the book.
This is one I think i read in the nineties. Humanity has very long lives and over the centuries society has changed. They opt for prolongation in mid puberty so everyone now appears to be like 14 or 15 but really several centuries old.
Space navy has senior admirals and raw recruit school leavers looking the same age.
Biggest enemy of humanity is a clone brotherhood out in the galaxy who are gradually taking over all man's worlds.
Protagonist is a heroic naval officer who, we learn early on, is deemed untrustworthy cos he is one of the clones but was raised by the goodies.
The clones occasionally do face on space battles but a lot of time they infiltrate planets and stealthily set up like monastic houses all over. They then raise clones until they outnumber the norms.
One trait is a cult of no personality, if you pick a red shirt from laundry piles then you are called Red for the day (or Blue / Green etc)
The goodies have an option called seclusion or reclusion when they get tired of centuries of strife against the clones. They stop taking immortality drugs and age and die normally.
Clones are getting nearer star system by star system to earth so naval hero goes on a long trek with his battle fleet to slow them down, he has some meetings with the clones, all with his face.
The navy he is in has a hierarchy reminiscent of the Nicholas Seafort books but all baby-faced.
I've done searches and Clone Brothers / Brotherhood in book titles gives a lot of results, I've gone through all these at various times but none are relevant
Somebody was looking for it in SFF Chronicles and I added onto his post hoping it might help some kind person to solve it for us both - No luck but a tad more info