C# checkedlist box items removal when a button is clicked

Hi,

The below is a snap from c# windows form application.

now, my concern is when I enter text in textbox and click add, it should check the following conditions:

1. check if the entered text contains a particular string(say @gmail.com)

2. after checking above condition, it should check if the entered text is present in listBox1, checkedListBox1, checkedListBox2. If it exists the elements should be prompt that element is present and prevent it from adding in the boxes. if it doesn't exists then it should add in all the 3 boxes.