Add ability to disable duplicate content block message

[praveenperera]

Added in #112 (#60).

I don't know if this is just me but there are some places on the website that this is the desired behaviour. I want to make sure that editing one content area automatically changes the other as well.

[jamonholmgren]

Perhaps we need an option, allow_duplicate: true or something?

Or duplicates: :allow | :warn.

[praveenperera]

Or duplicates: :allow | :warn.

I like this option, I think I'll have some time this week to try and make a PR.

[jamonholmgren]

That would be great @praveenperera !

[praveenperera]

I will try to take a look this weekend 😊