Just tested registering the .NET, and got the same error...
There is an IDN already registered somewhere that consists of some character variants, which visually appear different, but whose sequence is associated with the same .net IDN...
An IDN often consists of numerous domain labels (ie. aliases so to speak) ... when one registers an IDN, they also reserve all the associated variations at the same time.
On a related note, character variant tables can potentially change over time ... for most IDN registrations this will never be a problem, but is an issue to be aware of, since it's not clear how such changes are to be handled.
Good luck with the sale
Ron