The 1841 British edition added the subordinate conjunction that after “the Lord commandeth you”, thus creating a repeated that for this sentence: “the Lord commandeth you that when ye shall see these things come among you that ye shall awake to a sense of your awful situation”. This intrusive that was removed in the subsequent 1849 LDS edition. As explained under 1 Nephi 10:2–3, the original text had quite a few examples of the repeated that, but it had many more examples without it (for a complete discussion of the repeated that, see under that in volume 3).
Elsewhere in the text, when the verb is command, the subordinate conjunction that always precedes the finite-clause complement for that verb (127 times in the original text). And in actuality, here in Ether 8:24 we have the that, but it occurs after the intervening when- clause: “the Lord commandeth you … that ye shall awake to a sense of your awful situation”. Thus there is always a that, even if there is an intervening clause that delays the that- clause. Also see under Helaman 6:38 for another case (in the 1840 edition), but with a different verb, where an extra subordinate that was inserted, leading to another secondary instance of the repeated that.
Summary: Maintain in Ether 8:24 the single occurrence of the that right before the finite clause that complements the verb command (namely, after the intervening when- clause).