(1)    For:

<a href="abc.html" name="abc">abc.html</a><br>
<a href="ABC.html" name="ABC">ABC.HTML</a><br>

Names of <a> must be unique.

For:

Go to <a href="#def">def</a>
...
<span id="def">Some links:</span><br>
...
<a href="def.html" name="def">def.html</a><br>

Id and name share the same name space and should be unique.

For:

<a href="fgh.html">Two links:
 <a href="xyz.html">xyz.html</a>,<a href="xyz.html">xyz.html</a>
</a><br>

The <a> element cannot be nested.

Note that there is no other illegal code, according to the HTML 4.01 specification.