The compositor for the 1905 LDS edition created a dittography here, namely, that that. Interestingly, the compositor for the 1911 LDS edition, when he set the text from the third printing (in 1907) of the 1905 edition, retained this dittography. The 1920 LDS edition removed the repeated that.
Summary: Maintain the nonrepeated that in 3 Nephi 28:34.