Spacing between extant fragments of 𝓞 argues that here in Ether 15:17 the subordinate conjunction that may have occurred after “on the morrow” (in addition to before it). Yet the repeated that, if it were in 𝓞, may have been crossed out since it is not found in 𝓟. This case is discussed under Alma 58:14; from the discussion there, we can determine that the expression “it came to pass that on the morrow that …” is possible (as originally in Jacob 7:17), but the chances are greater that the subordinate that was not repeated after the prepositional phrase “on the morrow”. Here in Ether 15:17, the critical text will follow the earliest extant reading, namely, the reading in 𝓟 without the repeated that.
Summary: Maintain in Ether 15:17 the earliest extant reading (the reading in 𝓟) without the repeated that: “and it came to pass that on the morrow they did go again to battle”.