Freud jumps around in this essay but makes conceptual connections.
Freud notes that "uncanny" (unheimlich) means "that class of the frightening which leads back to what is known of old and long familiar." But he also notes that unheimlich (uncanny) must mean the opposite of heimlich, which means familiar. Logically following, uncanny must mean frightening which leads back to old and familiar but, being the opposite, it must also mean unfamiliar. In Part II of the essay, Freud will clarify that the uncanny is something that was familiar but has become unfamiliar via repression:
. . . nothing new or alien, but something which is familiar and old-established in the mind and which has become alienated from it only through the process of repression.
Freud then talks about repetition and the double. He suggests the soul is the first double. It is a second self, separate from the body. Like the paradoxical "uncanny" (familiar/unfamiliar . . . and threatening), the soul, as double, represents immortality but the death of the body. So, it is something that gives comfort and familiarity but its liberation from the body is also the harbinger of death. So, the soul carries implications of comfort and dread; it is familiar in its unfamiliarity because we don't really know what it is.
Freud gives a more ordinary example. When we see a number repeatedly in a series of connected events, it is familiar (because we have continued to see it) but it seems odd and maybe even threatening as if there is some unknown supernatural force at work; unless we are completely beyond superstitious implications.
Freud suggests that the uncanny is a return to something familiar that has become unfamiliar (or alienated) by repression. One example he gives is that neurotic men feel there is something uncanny about female genitals; that female genitals would be unfamiliar and threatening is the psycho-analyst's joke because that's where we all came from and therefore it is the first familiar (heimlich) home.
Some, not all, recurring repressions of what was once familiar are uncanny. Freud ends the essay saying there is no absolute conclusion.
Since this is Freud who often discusses unconscious desires, consider one more example. Kasper had a crush on his cousin when he was a child. Learning that this was taboo, he repressed this desire. But as an adult, he continued to be drawn to women who had his cousin's characteristics. He felt a familiarity (being drawn to them) but was unnerved by it because it was inexplicable to him. Kasper experiences an uncanny feeling: unnerving or even frightening, unfamiliar, but linking back to a familiar desire/event. This repeats because Kasper still has this desire and/or he, as most people do, seeks the familiar (even if that which we seek is consciously unfamiliar).