Bear with me, i am not the best at explaining things so this might be kind of confusing...I guess also spoilers for Minagoroshi if you haven't read that already...

Different iterations of the same "arc" exist. This is explained more in Massacre (Minagoroshi), specifically the manga/VN, where there's different "fragments" that get replayed in a randomized order. It is possible for two "Watanagashi/Meakashi" arcs to exist as different versions of the same "fragment", if Rika or another character make a decision that differs from the usual "route" (in this case, one version Shion tortures her to death, another Rika commits suicide to avoid the torture). There is an arc that exists where, in Atonement, Rena succeeds in blowing up the school because she kills Keiichi in the storage room. There also exist a version where she does not, which is the version we see in the series. Two iterations of the same arc.

The version of Watangashi/Meakashi we see in the series is NOT the first time Rika sees it. Once the series starts Rika has ALREADY experienced the arcs several times. That is why it is possible for Rika to remember what happened despite us, the viewer, only having experienced it for the first time and believe that Rika should not already know how she dies in it. Higurashi does not have a linear timeline, rather it is more like a constellation of different points that we jump to. Hope this makes sense lol.