runtime error 35602(key is not unique in collection)

experts, i'm experiencing the above error in the following code whilst trying to initialize my treeview:-

code:-
 Dim cat As Node, cat2 As Node, strLetters As String, adoAttendance2 As ADODB.Recordset, adoAttendance3 As ADODB.Recordset
    Dim I As Integer, strAttendance2 As String, strAttendance3 As String
    Conn
    lvwsLoad
    TreeView1(0).LineStyle = tvwRootLines
    TreeView1(0).Nodes.Clear

    strLetters = "ABCDEFGHIJKLMNOPQRSTUVXYZ"
     
     For I = 1 To 25
        strKey = Mid(strLetters, I, 1)
    Set adoAttendance2 = New ADODB.Recordset
    strAttendance2 = "select * from Players where Players.[Last Name] like '" & strKey & "%' and [Age Category] = 'Adult'"
    adoAttendance2.Open strAttendance2, objConn, adOpenStatic, adLockOptimistic
     
    Set cat2 = TreeView1(0).Nodes.Add(, , strKey, strKey, 1)
         'Start the loop from start
    If adoAttendance2.RecordCount > 0 Then
        While Not adoAttendance2.EOF
            With TreeView1(0).Nodes
        'Put the results together in the treeview
            Set cat = .Add(cat2, tvwChild, strKey & CStr(adoAttendance2.Fields("Last Name")), adoAttendance2.Fields("Last Name") & " " & adoAttendance2.Fields("First Name"), 2)
             adoAttendance2.MoveNext
           End With
           DoEvents
       Wend
    End If
    Next


 'error occurs on this line......
Set cat = .Add(cat2, tvwChild, strKey & CStr(adoAttendance2.Fields("Last Name")), adoAttendance2.Fields("Last Name") & " " & adoAttendance2.Fields("First Name"), 2)

the error occurs when strkey = U%....., i have four names that begin with the letter U, and if i  trap the error withe on error resume next, it returns only one name begining with letter u,........,
i also have 2 names begining with b and that works fine????????? i'm stumped!!
what does this error mean and how do i get rid of it...