xml-dev - duplicate enumeratons on attribute definitions

[Rob Lugt <roblugt@bigfoot.com>]

Looking through the XML 1.0 recommendation, it seems that duplicate
enumerations or notations on the same attribute definition are allowed.
e.g:-

<?xml version="1.0"?>
<!DOCTYPE el1
[
<!ELEMENT el1 EMPTY>
<!ATTLIST el1 Rob (one|two|two) #IMPLIED>
]>
<el1 Rob="two"/>

This appears to be valid apart from an interoperability constraint "the same
NmToken should not occur more than once in the enumerated attribute types

of a single element type".

Can anyone tell me if this is an oversight or by design?  And, if by design,

why?

Regards
Rob Lugt
Elcel Technology