Search result pagination loosing/doubling items

Question

Hi,
my client is running Portal 10.1.2.0.2 (Build: 139) and has a custom search portlet on a page.
It's set up to search for items of a specific custom item type in one page group only.
When users search the results sometimes (maybe always) lacks some items and doubles some (same item shown twice).
I am a fairly experienced Portal developer and have verified this myself.
If I search for some word X I get 408 document hits. If the portlet is set to paginate the result at 200 hits per page I get 15 items doubled. The last 14 on page 1 (1-200) are the first 14 on page 2 (201-400). The last item on page 2 is the first on page 3 (401-408).
Next I set pagination to 150 and search for the same word. Still 408 hits. This time 9 on the first page (1-150) showed up on the second page as well (151-300) (this time not last/first but 4 "single" items after/before the doubles). The last 12 items on page 2 was the same as the first 12 on page 3 (301-408). A total of 21 doubles.
So I set pagination to 500 (all on one page) and serached for the word again. This time I'm unable to find any doubles.
As the total number of hits was constant, this means that the first pagination did not show 15 items and the second pagination lost 21.
I have searched the forums and MetaLink but found nothing.
Has anyone heard of this before?
Kind regards
Tomas Albinsson
Stockholm, Sweden

Answer 1

OK, a month later...
The same search word now gives 410 document hits (more docs have been added).
A pagination of 200 now gives 15 doubles on page one (last 15 is the same as first 15 on page two) and one double on page two (last item is the same as first on page three).
I set up a new custom search portlet, restricted it to just find items of a certain subtype and let the user search for text and select a year (custom attribute of the item type). Same result as the original search portlet.
If I select another year I just get 26 docs. If pagination is set to 50 I see them all and no doubles. Changing pagination to 5 items per page produces two doubles. The last two on page 3 (items 14-15) are the same as the first two on page 4.
Compared to the complete list I can see that items 14 and 15 are wrong, they hide two docs in those places in the complete list.
Next I turned off the subtype restriction (ie all types are OK), leaving just search text and year for the user.
The same search word that gave 26 above now gave 40 hits.
A pagination of 15 was fine.
A pagination of 10 was fine.
A pagination of 8 gave two doubles: the last on page 3 (item 24) was the same as the first on page 4 (item 25) and the last on page 4 (item 32) was the same as the first on page 5 (item 33).
A pagination of 24 gave one double: the last item on page 1 (item 24) was the same as the first on page 2 (item 25).
A pagination of 23 gave two doubles: the last two on page 1 (items 22-23) was the same as the first two on page 2 (items 24-25).
Next I turned off the year custom attribute LOV restriction, leaving only the search text (for items in one page group).
A search for three words gave 42 hits.
A pagination of 17 gave one double, the last item on page 1 (item 17) was the same as the first on page 2 (item 18).
This seems fairly easy to reproduce. A custom search portlet restricted to items in a page group.
Find search terms that give a reasonable number of hits. Set the pagination higher then the amount first, then to something that gives a few pages. The doubles seem to show up at the page "borders" (last/first on a page).
As I have only one environment I'd be happy if someone could test this.
Kind regards
Tomas