Try this:
C#
IEnumerable<Control> ctls = this.Controls.Cast<Control>();
var txtBoxQuery = from c in ctls
where c is TextBox && ctls.Count(x => x.Text == c.Text) > 1
select c;
foreach (Control c in txtBoxQuery)
c.BackColor = Color.Red;
VB
Dim ctls As IEnumerable(Of Control) = Me.Controls.Cast(Of Control)()
Dim txtBoxQuery = From c In ctls Where TypeOf c Is TextBox And ctls.Count(Function(x) x.Text = c.Text) > 1 Select c
For Each c As Control In txtBoxQuery
c.BackColor = Color.Red
Next