καὶ ἐν τῷ διαβῆναι αὐτοὺς καὶ Ηλιου εἶπεν πρὸς Ελισαιε αἴτησαι τί ποιήσω σοι πρὶν ἢ ἀναλημφθῆναί με ἀπὸ σοῦ καὶ εἶπεν Ελισαιε γενηθήτω δὴ διπλᾶ ἐν πνεύματί σου ἐπ᾿ ἐμέ

My concern here is that the LXX here lost the meaning of the Hebrew in conveying the double portion of Elijah's spirit. In the Hebrew it would seem obvious he is talking of doubling a portion of Elijah's spirit, in the same way a firstborn son receives a double portion of inheritance.

But looking at an interlinear (Bible Bento) and the NETS LXX online (hosted by UPenn), it seems to lose the connotation of a double portion and tend toward a doubling of Elijah's spirit. Sirach 48:12 seems to approve of this reading as Elisha is said to have done twice as many miracles as Elijah.

Trying to look it up, I simply get that the LXX means to follow the Hebrew original. Sure, it meant to, but did it succeed?