I went to access management and then in user management in SDL Tridion 2011 SP1 I saw some duplicate entries of users. When I tried to delete these users through CMS then it showed an error "Invalid name in TRUSTEES".
If anyone has used any DB query to delete these Invalid entries from trustees table in DB, can you please tell we should delete it with all it's constraints or not.
Executing DB queries will void your warranty so I would not recommend that. Duplicate name entries can occur (in the database) when a user is deleted and later added again (deleted users stay in the database so the history will still have a correct reference), this should not lead to any issues (as the user name is not a unique key).
So for your mentioned error I would contact Customer Support as it might be something is wrong.