Was able to replicate this issue, would like to work on this

So, is overriding supposed to completely, "override" the profile, like replacing all the sources currently present with the one in JSON, or is it supposed to add the sources coming from JSON to the current list of sources.

For a scenario different than the one mentioned in the issue:
Exported JSON profile Pr1.json, has a source ~/Desktop/source_1

And a currently present profile Pr1 has a source ~/Desktop/source_2
So after importing 'Pr1.json', is it supposed to add the source from JSON under the new profile, or completely replace the existing sources.

As per current behavior, the new sources get appended to the list of existing sources under a profile upon overriding. Which explains the duplicate entries mentioned in the issue.

So are we just looking to avoid duplicate entries, or completely replace the existing sources under a profile

Override existing settings suggests "all settings" should be overridden. If only "Sources" are threaded the other way (what about "exclusions"? for consistency, the same should be applied to them), e.g. to give a user an ability to add extra "sources" into the existing profiles, there could be an extra import settings to treat is that way.

Nevertheless, I don't know if the aforementioned idea would be useful for anybody, and in my opinion, the default behavior would be to override "all backup settings" (including sources and exclusions).

@m3nu should I work on this