Option Strict On

Public Class mainForm


    Private Sub addButton_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles addButton.Click

        Dim counterInteger As Integer = 0I
        Dim matchBoolean As Boolean = False

        Do Until counterInteger = fruitComboBox.Items.Count
            If fruitComboBox.Text.ToLower = CStr(fruitComboBox.Items(counterInteger)).ToLower Then
                matchBoolean = True
            End If
            counterInteger += 1
        Loop

        If matchBoolean = False Then
            fruitComboBox.Items.Add(fruitComboBox.Text)
        Else
            MessageBox.Show("This item already exists", "Error:Match", MessageBoxButtons.OK, MessageBoxIcon.Information)
        End If

    End Sub
End Class