Lens - No duplicate title check / warning

[ThomThomson]

Kibana version:
7.7

Browser version:
Chrome 81

Original install method (e.g. download page, yum, from source, etc.):
from source

Describe the bug:
When saving a lens visualization, duplicate titles are allowed without a warning.

Steps to reproduce:

1. Create and save a lens visualization
2. Create another new lens visualization and save it with the same title

Expected behavior:
The visualize editor gives this warning when a duplicate title is entered. Lens should give a similar warning to prevent many visualizations sharing the same name.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)