Only 1 ContactID can be added to each table

Hi - I use Access 2010.

I have a form which I use to add data to a table.  And I have another table which has all my Contacts Details including a field unique to them called ContactID.

Basically, when a user starts to fill in the form they have to enter their ContactID.  If they have already filled in the form before and try entering their ContactID again, it will display a message saying that their ContactID has already been used and so they can't carry on which is great and this is the code I use.

---------------------------
if dcount("*","tbl1", "ContactID=" & me.cblContactId) > 0 then
   msgbox "Contact ID already exists! I'm sorry but you have already completed this form before"
   cancel=true
end if
---------------------------

However, I have since added a search form which allows a user to click on a button within the main form which opens up another form called "frmContactsSearch".  This form basically has a list box inside which displays all Contacts details (Contact ID, Title, First Name, Last Name, DOB, Telephone).  All the user has to do is click on their name, and click a button called 'input to form' which basically puts all their details into the original form.  this is the code that sits within the 'on click' event.

--------------------------------------------------
Private Sub btnInput_Click()

On Error GoTo Err_btnInput_DblClick

Forms("frm1")![cboContactID] = ListBox.Column(0)
Forms("frm1")![cboTitle] = ListBox.Column(1)
Forms("frm1")![cboFirstName] = ListBox.Column(2)
Forms("frm1")![cboLastName] = ListBox.Column(3)
Forms("frm1")![cboDOB] = ListBox.Column(4)
Forms("frm1")![cboTelephone] = ListBox.Column(5)
Me.Refresh

DoCmd.Close

If IsNull(ListBox.Value) Then

DoCmd.Close
DoCmd.OpenForm ("frm1")

End If

Exit_btnInput_DblClick:
    Exit Sub

Err_btnInput_DblClick:
   
    Resume Exit_btnInput_DblClick

End Sub
------------------------------------------

I basically need it to do the same thing as the code at the top that when a user selects their name from this searchform, if when they click the button 'input to form' that ContactID has already been used in the form, it will display a message box informing them that that ContactID has already been used and will not allow them to add it.