Notes/Domino 6 and 7 Forum : Why server duplicates some documents???

Hi there!
I create contracts.nsf (for web users only) with contracts form which contains contracts information. Every document has unique contract id, so i add input validation @formula for ID field:

fID:=@DbLookup("":"NoCache";@ServerName:@WebDbName;"contracts view";@thisvalue;1;[FailSilent]);
@if(@IsNewDoc&fID!=""; @Failure("Contract whith this ID allready exists in DB!");@Success)

When form is opened for new documents ID field will appear for filling, when doc is opened for modifying, ID will disappears and computed text will show content of ID field. So user could only first time fill that field.
It is working perfectly....
I try to fill same contract with a same ID two times, ups message "Contract whith this ID allready exists in DB!".

BUT yesterday users called to me and said "Hey i found two contracts with same ID, which of them is mine...". I found some documents in DB with duplicate IDs. I tried to fill duplicate data through browser but validation is working.

I have analyzed all duplicate data:
1.All duplicate documents has only one copy (not two, not three);
2.Copy of duplicate has same data and same created date as originall;

row | ID | created date
------------------------------------
1 | 111 | 01.11.2003 15:22:01
2 | 111 | 01.11.2003 15:22:01
3 | 222 | 02.11.2003 11:01:23
4 | 333 | 03.11.2003 14:30:10
5 | 444 | 04.11.2003 19:20:00
6 | 555 | 05.11.2003 09:20:20
7 | 555 | 05.11.2003 09:20:20

I think duplicates created by server, not by users. Is that servers BUG???

Please help...

thanks..
Farruh