I don't think this is made clear anywhere in any book but my guess is that Glorfindel of Rivendell is different to Glorfindel of Gondolin (maybe a look at some family trees in the Silmarillion or appendicies would confirm this) They are perhaps related. Perhaps he is not reincarnated but it is the other Glorfindel which returns for the battle of the five armies. Another possiblity is that it is the same person but he returns changed from the encounter with the Balrog much as Gandalf did, returning as white! Gandalf in the Fellowship of the Ring refers to him as an elf-lord when speaking to Elrond.
I'm trying to help but I probably haven't!
Airetauriel
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