Here in the printer’s manuscript, Oliver Cowdery initially repeated the subordinate conjunction that; he wrote “that even since the days of Abraham that there hath been many prophets”. (For the occurrence of even in 𝓟 rather than ever, see the preceding discussion.) As discussed under 1 Nephi 10:2–3, there are examples in the original text of the repeated conjunction that; but of course there are also examples without the repeated that. In each case, the critical text will follow the earliest textual sources. Here in Helaman 8:19, Oliver’s crossout of the repeated that appears to be virtually immediate (there appears to be no change in the level of ink flow for the crossout). The original manuscript, not extant here, probably lacked the repeated that.
Summary: Maintain the lack of the repeated subordinate conjunction that in Helaman 8:19 (the corrected reading in 𝓟, the earliest extant source for this part of the text).