The 1879 LDS edition replaced this in this passage with the; the original this was restored to the LDS text in the 1920 edition. The demonstrative this occasionally occurs before names, although all other examples lack the word same (see the examples of “this ” listed under Alma 11:21); note, in fact, the nearby example in this chapter: “now this Melchizedek was a king over the land of Salem” (Alma 13:17). There are no examples in the original text of “the same ”, while there is one example of “this same ” (namely, here in Alma 13:15). The critical text will maintain the unique occurrence of this same in this verse.
Summary: Maintain in Alma 13:15 the earliest reading: “and it was this same Melchizedek to whom Abraham paid tithes”.