Dashboard - Don't handle having 3 the same visualisation columns in the widget after saving

### Environment

SaaS (https://sentry.io/)

### Steps to Reproduce

1. Create widget
2. Create 3 the same columns in the vizualisation section
3. Hit save

See Sentry issue: https://sentry.sentry.io/issues/7050468989/events/67207eae10054f9ca068593a4bcbcd6f/

### Expected Result

Saved widget shows correctly

### Actual Result

Internal error. Please try again.
https://demo.sentry.io/dashboard/127293/?project=5808623&statsPeriod=7d

### Product Area

Dashboards

### Link

_No response_

### DSN

_No response_

### Version

_No response_

Reproduce the failure in the SaaS dashboard by creating a widget with three identical visualization columns and saving it. Trace the dashboard widget save flow from that reproduction, then verify that the saved widget displays correctly without the internal error.

Written by the indexing model from the issue text.