XSD: Enumeration duplicates - xsd

Question

Recently, I discovered that one of our xs:enumeration types included the same value twice:
<xs:simpleType name="typ-TypeCodeRequest">
<xs:restriction base="xs:string">
<xs:enumeration value="B1"/>
<xs:enumeration value="B2"/>
<xs:enumeration value="B2"/>
<xs:enumeration value="B3"/>
<xs:enumeration value="B4"/>
<xs:enumeration value="B5"/>
</xs:restriction>
</xs:simpleType>
Now, an external partner complained about it, claiming that "that cannot ever work". This confused me somewhat, since my tries to find out wether or not duplicate entries in enumerations are allowed or not - even if pointless - were fruitless.
This was not detected as wrong by any validation, and did not result in any problems when generated into code and used with Apaches CXF framework. Are we handling this issue too lax, or is the external partner too strict?

Answer 1

Strictly speaking, the gist of your problem is really in clarifying the context in which one said that cannot ever work.
In terms of the XSD spec, your fragment is valid - so that person is wrong. Duplicate enumerations are annoying to read and most likely indicate a bug, due to a typo which maybe misses one of possible values... still, perfectly valid.
The XML Schema spec, in both 1.0 and 1.1 (section 4.3.5) has no restriction placed on the uniqueness of the enumerated values. It's all about It is an ·error· if any member of {value} is not in the ·value space· of {base type definition}.
Interestingly enough, both specs could've placed constraints in the "schema for schema" to ensure uniqueness... but none did.
To expand on this... It is easy to place redundant constraints; a sequence of enumerated values could also be written using regex patterns. For e.g.:
<?xml version="1.0" encoding="utf-8" ?>
<!-- XML Schema generated by QTAssistant/XSD Module (http://www.paschidev.com) -->
<xsd:schema targetNamespace="http://tempuri.org/XMLSchema.xsd" xmlns="http://tempuri.org/XMLSchema.xsd" elementFormDefault="qualified" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<xsd:element name="test" type="test"/>
<xsd:simpleType name="test">
<xsd:restriction base="xsd:string">
<xsd:enumeration value="y"/>
<xsd:enumeration value="n"/>
<xsd:pattern value="y|n"/>
</xsd:restriction>
</xsd:simpleType>
</xsd:schema>
In this case, the pattern has no use... yet, the XSD spec does not flag this as wrong (even though it is superfluous, since the enumeration always wins).
Maybe that person's problem is caused by some program that binds XSD to something else... and that program is creating duplicate entries, on the assumption that enumerated values should be unique (which is a wrong assumption).
If I would be you, I would simply fix the XSD; and ensure that you're using some XSD static analysis tooling to validate that it doesn't happen in your releases (even though is valid).

Answer 2

I would suggest that their complaint is more politically rather than technically motivated.
However, it is clearly incorrect and if you did try to convert this enum to a type in say, c#, you'd be unable to cleanly do it.
eg: won't compile:
enum Color
{
White = 0,
Black = 1,
Orange = 2,
Orange = 3
}
So if it's clearly incorrect why not create a new version of your schema?