[DOCS] Inconsistent treatment of entries in new docs build

I made a change in 3c9d39848 that inserted these index entries:

    <indexterm>
     <primary>median</primary>
    </indexterm>
    <indexterm>
     <primary>median</primary>
     <seealso>percentile</seealso>
    </indexterm>

The new docs toolchain doesn't complain about this, and it produces
the output I wanted:

    median, Aggregate Expressions
        (see also percentile)

However, "make oldhtml" complains:

Processing HTML.index...
2705 entries loaded...
collateindex.pl: duplicated index entry found: MEDIAN
1 entries ignored...
Done.

and it doesn't emit the "see also" entry.

So, is what I wrote valid SGML?  If so, can we fix the older build
process to cope?  If not, can we fix the new process to complain?

The thing that is bothering me is that I back-patched this change without
realizing that it wouldn't work in the back branches.  This won't be the
last such mistake, if we can't persuade the two toolchains to accept the
same input.

            regards, tom lane