Ethernet loops and dual protocols

I have the same PC connected to the same Ethernet segment (one hub) via two NIC’s.
One NIC has TCP/IP loaded onto it the other has the IPX/SPX protocol loaded onto it. I know that loops are not allowed on Ethernet, and this is a physical loop configuration. However, it has been suggested that it is not a logical loop. In other words an Ethernet frame containing an IPX/SPX addressed code cannot go through OSI layer 3 to the application level on the card running TCP/IP and therefore no harm is done.
On the other hand I suspect the constraint is an Ethernet one because the damage done is at OSI layers 1 and 2. My problem is that I cannot find anywhere the reason why looping is not allowed on an Ethernet segment.

Can anyone help please? Ineed a solid argument to validate my hunch.
Thank you.