Portals defy physics on so many levels that it's hard to get in to. Can I just say the "No Cloning Principle" is at play here and be done with it?

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Whether or not something is moving is a matter of perspective, not a universal truth.
Am I sitting still at my computer, or whirling around the sun at 30 km/s?
Why not both?

To the portal, the people are moving relative to it.

If you suggest the portal should plop the people out, then where does it plop them? They'd all enter the portal at the same location and if they leave with no speed, then they manifest as a plane, not even considering what the next person through would do.

If the argument is that there is some lack of motion which dominates the relative motion of the portal and people, then even the first person is problematic as the first molecules of theirs that come through the portal do not move away. Then the next molecules are put in the same space as the first, and we have real problems.