Just like I said in the title, it is possible to open many portals at the same time to the same destination (even though it should be impossible according to the feature guide). Only the portal that has been opened the latest actually closes automatically alongside the destination portal, but the portals that have been opened prior stay open and can teleport to the destination even when the destination portal is already closed. The only way to close them is by flicking the lever on the source portals (not on the destination), or by deleting the destination portal (which breaks the source portals too, and they need to also be deleted and created again)
Minecraft version: 1.21.8.