In the myth of Helen of Troy, there has been noted a story from Egyptian mythology of Helen coming to Egypt and staying there, her double being sent on to be imprisoned and have a war fought over. Does anyone know of any other mentions or references of this double? In Egyptian mythology, a soul has three parts, and it is not uncommon for one of those parts to be separated and create that double. A similar thing has been noted in Viking mythology, but I have not come across it in any Greek or Roman sources.
Not a exact answer, as this is probably where you got it from, but, here goes!
At least three Ancient Greek authors denied that Helen ever went to Troy; instead, they suggested, Helen stayed in Egypt during the duration of the Trojan War. Those three authors are Euripides, Stesichorus, and Herodotus. In the version put forth by Euripides in his play Helen, Hera fashioned a likeness of Helen (eidolon, εἴδωλον) out of clouds at Zeus' request, Hermes took her to Egypt, and Helen never went to Troy, spending the entire war in Egypt. Eidolon is also present in Stesichorus' account, but not in Herodotus' rationalizing version of the myth.
Herodotus adds weight to the "Egyptian" version of events by putting forward his own evidence—he traveled to Egypt and interviewed the priests of the temple (Foreign Aphrodite, ξείνης Ἀφροδίτης) at Memphis. According to these priests, Helen had arrived in Egypt shortly after leaving Sparta, because strong winds had blown Paris's ship off course. King Proteus of Egypt, appalled that Paris had seduced his host's wife and plundered his host's home in Sparta, disallowed Paris from taking Helen to Troy. Paris returned to Troy without a new bride, but the Greeks refused to believe that Helen was in Egypt and not within Troy's walls. Thus, Helen waited in Memphis for ten years, while the Greeks and the Trojans fought. Following the conclusion of the Trojan War, Menelaus sailed to Memphis, where Proteus reunited him with Helen. https://en.wikipedia.org/wiki/Helen_of_Troy
So we see that some authors disagree, but a lot more do agree. I'll just give you Herodotus's account.
Correcting Homer: Paris and Helen in Egypt
The Egyptian priests say Paris and Helen were blown off course on their way to Troy and shipwrecked near a shrine of Heracles in Egypt. The servants of Paris took refuge at the shrine and denounced him as a rapist to the local Egyptian official, Thonis. Thonis had Paris arrested and brought before King Proteus at Memphis (113-14). Proteus conducted an investigation and pronounced Alexander guilty; he kept Helen in Egypt and sent Paris home (115). Citations of Homer prove that he was aware of this version (116). This incidentally also proves that the Cypria, a poem of the Epic Cycle, is not by Homer (117). All this leads Hdt. to ask the Egyptian priests whether in their opinion the Trojan War really happened. Menelaus himself told their predecessors that it did, but that the Greeks only learned the truth, that Helen was in Egypt, after the fall of Troy (118). Menelaus went to Memphis to retrieve Helen and was well received by Proteus; but he later fled Egypt in disgrace after sacrificing two children to allay contrary winds (119). Hdt. believes this version and supports it with an argument from probability: if the Trojans had had Helen, they would surely have given her back rather than allow their entire city to be destroyed (120).
And, we see that Herodotus does give a account of Helen being on Egypt.