This bug is probably the same issue, but the reporter
didn't give any info and thus it was closed.:
https://sourceforge.net/tracker/?group_id=31577&atid=402788&func=detail&aid=726225

Here's an example of how it's broken:

http://twistedmatrix.com/users/roundup.twistd/twisted/issue?%3Acolumns=id%2Cactivity%2Ctitle%2Ccreator%2Cassignedto%2Cstatus&%3Afilter=status&status=-1%2C1%2C2%2C3%2C4%2C5%2C6%2C7&%3Apagesize=50&%3Astartwith=0&%3Asort=activity&%3Asortdir=on&%3Agroup=topic

Notice that it's displaying multiple "documentation"
groups -- there seems to be no good reason for it doing
that, at least that I can see.

Similar things happen when you group on nosy.

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Which backend are you using? Note there's an outstanding bug 
filed against "RDBMS backends can't sort on multilink". I'd say 
there's a good chance you're running into this bug?

The bug is fixable, but the fix will be slow (ie. it'll be done in a 
post-SQL step). I've not had time to address it, since it's not a 
serious bug (IMO :)

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No, that bug is totally different. I'm using the regular
BSDDB backend.  Please see the URL I posted in my last
comment, it shows the behavior.

The problem isn't in what's being displayed in the drop down
listboxes, it's after I select "topic" in group-on and
redisplay, the actual list of issues is displaying multiple
"documentation" sections. the issues in them are distinct,
i.e., it's not displaying the same issue multiple times,
it's just unnecessarily splitting up a single group into
multiple groups.

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I've changed the *dbm backends so they sort multilinks by 
contents, rather than by length (as specified in Ping's design) .. 
this should result in more sane grouping.

Only applied this to 0.6 at this time.