Duplicates/Loops in Summary Buffer

[Graham Murray <graham@barnowl.demon.co.uk>]

Using the latest CVS, I am seeing a problem in the Summary Buffer (so
far only with nntp backend.) Some articles appear twice, either
immediately following one another, or the 2nd comes after replies to
the first (the sub-thread is not repeated, only the one article.)
This introduces a "loop" where trying to enter the 2nd occurrence
takes you (in the summary buffer) back to the first. So you cannot use
<space> or 'n' to leave the article, as these cause it to be
displayed again. The first occurrence is marked 'R' but the 2nd one
has no mark (unread). 

Here is an example.
O  +   148                           [  34: Fishfood            ] 
O  +   142                              [  25: Simon               ] 
O  +   142                                 [  11: Alexis Manning      ] 
R. +   148                                    [  22: Fishfood            ] 
 . +   148                                    [  22: Fishfood            ] 
O  +   145                                 [  42: dormouse            ] 

Are there any diagnostics I can take to help identify the cause?