It seems daft, but I think when you use the same bind variable twice you have to set it twice:
cmd.Parameters.Add("VarA", "24");
cmd.Parameters.Add("VarB", "test");
cmd.Parameters.Add("VarB", "test");
cmd.Parameters.Add("VarC", "1234");
cmd.Parameters.Add("VarC", "1234");
Certainly that's true with Native Dynamic SQL in PL/SQL:
SQL> begin
2 execute immediate 'select * from emp where ename=:name and ename=:name'
3 using 'KING';
4 end;
5 /
begin
*
ERROR at line 1:
ORA-01008: not all variables bound
SQL> begin
2 execute immediate 'select * from emp where ename=:name and ename=:name'
3 using 'KING', 'KING';
4 end;
5 /
PL/SQL procedure successfully completed.