There is no substantial evidence that the 1841 printer ever tried to edit the Book of Mormon text, so the introduction of in here in 2 Nephi 11:5 seems to be a typo, undoubtedly influenced by the occurrence of his before both grace and justice, which thus triggered the repetition of the preceding in. The RLDS text derives from the 1840 Cincinnati (Nauvoo) edition, not from the 1841 British edition; thus the original reading without the extra in has been maintained in the RLDS editions.
The introduction of the second in suggests the possibility that “the great and eternal plan of deliverance from death” might depend on only God’s justice, power, and mercy, not his grace. By removing the intrusive second in, the possibility of such a misreading is reduced.
Summary: Maintain the reading of the original text by removing the 1841 typo that inserted the preposition in before his justice.