$this->db->insert related problem

[eluser]Aziz Chouhan[/eluser]
hi all
i am working on codeigniter project since few days.
i have worked on all db related functions like select ,delete , update and insert, and these all worked very well.
but now i am facing a problem in db->insert();

this is my code which i written in a model function.

function update_hit_count($field_name,$field_id)
{

$data = array( 'field_name' => $field_name , 'field_id' => $field_id, 'hit_count' => '1' );
$this->db->insert('hit_count', $data);

}

when i called to this function with args as ('teacher',42)
it results 1 row as correct result
and also 3 or 6 rows as bad result with ('teacher',0,1)

i don't know where i m doing mistake
-------------------

i have also checked

$this->db->last_query();

which returns proper query

and also
$this->db->affected_rows()

which returns 1 row affected


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can anybody help me on this issue
its really very urgent