Issue with interactive symbols with same `name`

``` python
In [47]: s0 = bz.Data(1, name='s')

In [48]: s1 = bz.Data(2, name='s')

In [49]: s0
Out[49]: 1

In [50]: s1
Out[50]: 2

In [51]: s0 + s1
Out[51]: 4
```

Start by reproducing the interactive example with two bz.Data objects sharing the name "s" and inspect the Data and expression entry points. Add a regression test for the demonstrated operation, then verify that combining the two values produces the intended result rather than treating the same name as the same symbol.

Written by the indexing model from the issue text.