What you are encountering is one of the differences between XML and SGML. ISO 8879 (the SGML standard) does not allow two attributes in the same ATTLIST to have the same value.
The reason for this were the "tagging" shortcuts that were allowed in the SGML standard. When SGML came about, there were no sophisticated tools to tag data (such as Adept). So for those who tagged with an ASCII editor, SGML allowed things like tag minimization (for omitting the end tags on some elements) and for attributes, the author only had to enter the attribute value, but not the attribute name. So even though SGML requires a DTD, if the document uses minimization how would the parser know which attribute value goes to which attribute.
There is one exception to this rule. NUMBERs can be duplicated, any other attribute value must be unique.
This is not an issue in XML where the W3C standard does not allow any tag minimization, every element must have an end tag mark of some kind. The standard also requires that attributes be listed in the start tag with attribute name and value. The rationale for this is XML does not require a DTD (or schema), so the parser needs to have values (even if they are defaults).
I love traveling, except when I can't get an email connection. 😞 Anyway I agree with Gabriel and her editors. I've been working with the 0 | 1 choices for (a long time) and I have gotten them so muddled in my brain that I have to keep a cheat sheet handy so I know which is which.
But it was a way to work around the problem of no duplicate attribute values in a single ATTLIST. Imagine what things would be like if we could not have used the NUMBER.
While I am on this trend. Thought I'd pass out a little comment I heard the other day while I was at this conference. Seems there is a group that has a lot of data in SGML and are desirous to convert to XML, but they are afraid the conversion will be too difficult. I got the impression the individual didn't know enough about either and thought there were insurmountable problems because of the INFERIORITY of SGML. Though in a later discussion, it seems his concern was converting Dorkbook, errr, Docbook variant SGML to S1000D XML, which could be "fun" (sick).
SGML has some limitations, but from a tagging perspective, I can do the most of the things in SGML as I do in XML "IF" (you know Kipling's key word) the final product of the SGML will be processed as XML. In some instances, SGML is even more powerful than XML (I still miss my 'exclusions' [but not my 'inclusions']). For those of you who have not worked with SGML and don't know what an 'inclusion' or 'exclusion' is, post a note and I'll go into the gory, graphic details.
I do apologize for mixing your gender. As several members of Adepters may tell you, in English Lynn can be either a name given to either a boy or a girl, though it is more commonly a girl's name, though like you I am not.