I can not clear comboBox

Hello

I have tow comboBoxes, the first one get its items from DataGridView  by condition, and the second one is empty, then when i select from the first comboBox, then the items of second comboBox must changed according to the text selected in the first comboBox, also the second comboBox, gets its items from DataGridview by another condition.

Now this procedure is done correctly, but the problem is: when i change selection in the first comboBox, the second comboBox add its new Items and add them to the old Items.

Although I use comboBox2.Items.Clear() within comboBox1_SelectedIndexChanged(...).

in other way, i use comboBox2.ResetText(), but no resolve.

also i use: while(comboBox2.Items.Count>0){ comboBox2.Items.RemoveAt(0);}, but no resolve too.

can you help me, why Method Clear() did'nt work?

thank you