Strange issue on domino ldap - accumulation of groups

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C#

ASP.NET

.NET4

Hello,
I need help at following problem:
I query all groups of a user and show them in a listbox. This works well with Active Directory. But on domino server the same code accumulates the groups for different users.

For example: User1 is member of groupA and groupB, User2 is member of groupA and groupC

The first query for User1 is correct, listbox contains
groupA and groupB.

The second query for User2 contains groupA, groupB, groupA and groupC.
The groups of the first query are also returned.

This issue is only on domino server (it's not a listbox failure, the second query return 4 groups). This behaviour continues - another query for User1 now returns 6 groups and so on.

Has someone an idea?
Thanks!
Matt