I think what happened here is conflating "alternate" and "understudy". An alternate, like Javier, goes in on scheduled days (unless he himself is sick). He doesn't have another role in the show aside from alternating in the lead roll. An understudy typically has another, smaller role in the show but can swap into a principal role if someone is sick or has to take off for whatever other unexpected reason. So Jon Rua plays Charles Lee and is in the ensemble, but also understudies Hamilton and has had to cover for Javier when he was sick on his scheduled performance days.
So Rachel Tucker has an understudy, but not a scheduled alternate.
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