I've noticed this happens sometimes, and it seems to only happen if the chunks on the other side of the portal aren't already loaded when the item gets thrown through. I've been playing around a lot with lava mining from the nether and bringing it into the overworld and as long as the nether chunks are kept loaded (for example) duplication doesn't occur. I think it's actually a vanilla bug. Portal duping has been around a long time.

How I prevent it from happening is a loop using a belt system on either side of the portal that does nothing but toss an item back and forth to keep the portal live and the chunks loaded. As long as you have an item going through less than 15 seconds apart the chunks won't unload

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if someone can find a way to reproduce this reliably that would be great. Even better if done in vanilla, because that would mean this isn't even our issue.

Now that Immersive Portals functions with Create I've been playing around some more with this and the bug hasn't surfaced. My money is on this being a vanilla bug. Portals created with IP automatically keep some chunks loaded on the other side of a portal after all.

Immersive Portals functions with Create

May I ask how you made them compatible with each other? I get a warning each time I load the game with both mods and whenever an immersive portal is in view, the world you're in switches to that of the portal.

The warning is still displayed, but the two mods work well together if IP is configured correctly. Go into IP's client side settings and enable Compatibility Render Mode.