Users can't choose to add only one slice on Dashboard when there are multiple slices with the same name

[FrederichCheng]

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• I have checked the superset logs for python stacktraces and included it here as text if any
• I have reproduced the issue with at least the latest released version of superset
• I have checked the issue tracker for the same issue and I haven't found one similar

Superset version

0.18.5

Expected results

Users should be able to select only one slice

Actual results

When users select a slice, other slices with the same name will also be selected.

Steps to reproduce

1. Create two slices with the same name
2. Go to Dashboard
3. Add slice from Dashboard Controls
4. Select one slice among the slices with the same name