The Achille's Heel folk motif, i.e. invulnerability except in one spot, is a very common one worldwide. It appears as motif number Z311 in Thompson's Motif-Index of Folk-Literature, where an incomplete list of attestations of the motif is given, ranging from Irish to African folklore, passing through Jewish, Eskimo and Siberian tales.
With such widespread folk motifs it is always hard to argue for a common (pre-historic?) origin, independent developments, or mutual influences. There is however a set of Indo-European myths concerning imperfect invulnerability which bear such striking similarities that they have led scholars to conjecture a common origin, or at least strong mutual influences. The tales of Greek Achilles, Norse Sigurd and Balder, and Ossetian Soslan belong to this category.
The last one in particular seems to be the link that brings the remaining three together, to the point that Fridrik Thordarson, in his paper Die Ferse des Achilleus‐ein skythisches motiv? (in German) proposes a Scythian (the Ossetians being descendents of the Scythians) origin of the myths of Achilles and Sigurd. At the same time, Georges Dumézil in his book Loki draws a very convincing parallel between the tales of the deaths of Balder and Soslan, and argues for a common Proto-Indo-European origin.
Also John Colarusso in his Peoples, Languages and Lore agrees on the similarities between the four myths. He writes on Soslan (North-West Caucasian Sosruquo):
He [Sosruquo] is born aflame from a rock and tempered by
the god of the forge [...]. He is vulnerable in his knees or thighs, where he was held by the smith’s tongs, and in this localised soft spot he shows a close parallel with Greek Achilles and Norse Sigurd.
And this is what he has to say on Syrdon (the Ossetian homologue of Norse Loki) who, much like the latter is responsible for the death of Balder, causes Soslan's death by finding out his only weakness:
A figure restricted to the Ossetian corpus is that of Syrdon, [...] a trickster figure similar to Norse Loki. As Loki resents the gods for
their glory and loftiness, so Syrdon is maddened by the fame and arrogance of the Narts [i.e. the Ossetian heroes]. [...] The Narts tolerate Syrdon’s mischief up to
a point [i.e. the death of Soslan], just as the Norse gods tolerate Loki’s, up to a point [i.e. the death of Balder].