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GK librarian with shrouding in Ghost Knight unit with stealth.
Stealth USR: "All of the unit's cover saves are improved by +1." So does the libby/ghost knight combo work?

BGB FAQ:
Q: If only some of the models in a unit have the Stealth
special rule, does the whole unit benefit from the +1
cover save? (p76)
A: Yes. In effect the ones with the Stealth special rule
ensure their colleagues also find good places to hide.

No is my answer: a unit cannot have the same USR twice.
@ grakmar: How this Faq is relevant in the subject?

Agreed, the FAQ cited by grakmar is not relevant.

Yah, I don't think multiple things of the same special rule stack.

A librarian gives the unit Stealth USR.

A Ghost Knight unit has the Stealth USR.

Stealth grants a +1 to the units cover saves.

The unit has the Stealth USR twice, which is redundant.

Ah, apologies. I didn't realize the Ghosts already had stealth. I thought this was a simple "Does an IC with Stealth give it to a squad?"

I'm in agreement with the other posters. Having additional copies of Stealth does nothing. There's already plenty of examples (squads with camo cloaks, Eldar Rangers, etc) that have redundant copies of Stealth and do not get stacked.

If you are talking about the Eldar Pathfinders, the codex specifically states they get an additional +1 benefit which stacks.