Loading holidays twice ignores conflicts and generates duplicate entries

[araujoarthur0]

Describe the issue
When there are conflicts in sourcing holidays, we are not clear on the message.
The duplicates remain in the table after clicking, but disappear after closing the window.

To Reproduce
Steps to reproduce the behavior:

Once you open again, they are overriden, they just seem like they were duplicated in the table.

Expected behavior

1. We can override them, but we should inform the user when they click on "Source selected holidays" if there's a conflict, and wait until they choose "yes".
2. There should not be duplicates on the table after selecting.

Environment

Version: 3.0.1-dev
Electron: 11.5.0
Chrome: 87.0.4280.141
Node.js: 12.18.3
OS: Windows_NT x64 10.0.22621

[araujoarthur0]

Now the checkboxes are properly set.
But it would still be interesting to have a pop-up alert when one ignores the conflict and unchecks it.