DataRelation - These columns currently don't have unique values

I have a database with tables like this:

TableA:

ID
--
A
B
C

XrefAB table:

ID   Name
--   ----
A    Alpha
B    Bravo
C    Charlie

TableB:

Name    Data
----    ----
Alpha   1
Bravo   2
Charlie 3

I have a DataGridView displaying TableA.
And I have a TextBox to display Data from TableB.

DataGridView
+-----+         TextBox
|  A  |        +------+
|  B  |        |      |
|  C  |        +------+
+-----+

User selects a line from DataGridView, and TextBox displays corresponding data.

For example, User selects line A. The XrefAB table says A maps to "Alpha". TableB name "Alpha" corresponds to data "1". Display "1" in TextBox.

This I can accomplish as follows: (I'm using an Oracle database. Assume 'connection' is a connection to the Oracle database.)

DataSet ds = new DataSet();
OracleDataAdapter adapterTableA = new OracleDataAdapter("Select * from TABLEA", connection);
OracleDataAdapter adapterXrefAB = new OracleDataAdapter("Select * from XREFAB", connection);
OracleDataAdapter adapterTableB = new OracleDataAdapter("Select * from TABLEB", connection);

adapterTableA.Fill(ds, "TableA");
adapterXrefAB.Fill(ds, "XrefAB");
adapterTableB.Fill(ds, "TableB");

DataRelation drA2Xref = new DataRelation("TableAXref", 
      ds.Tables["TableA"].Columns["ID"],
      ds.Tables["XrefAB"].Columns["ID"]);
ds.Relations.Add(drA2Xref);

DataRelation drXref2B = new DataRelation("Xref2TableB", 
      ds.Tables["XrefAB"].Columns["Name"],
      ds.Tables["TableB"].Columns["Data"]);
ds.Relations.Add(drXref2B);

BindingSource xrefBS = new BindingSource();
BindingSource textBoxBS = new BindingSource();

xrefBS.DataSource = ds;
xrefBS.DataMember = "TableA";

textBoxBS.DataSource = xrefBS;
textBoxBS.DataMember = "Xref2TableB";

dataGridView.DataSource = ds;
dataGridView.DataMember = "TableA";

textBox.DataBindings.Add("Text", textBoxBS, "Data");

and this works OK.
Now I change the XrefAB table to:

ID   Name
--   ----
A    Alpha
B    Alpha
C    Charlie

Suppose user now selects from the DataGridView row B.
B maps to "Alpha".
"Alpha" maps to "1".
I'd like the TextBox to display a "1".

But it doesn't. Instead the whole thing blows up with the operating system complaining my XrefAB table Name column does not have unique values. The error message thrown is "These columns currently don't have unique values".

Why won't this work, and, how can I make this work?