Avoid adding a duplicate recipe

[mudnug]

Is your new feature related to a problem? Please describe.
Avoid adding a duplicate recipe. There may be scenarios where A person forgets they have already imported a recipe or the extension fails to open a tab and a second attempt is made to import the recipe.

Describe the solution you'd like
If I try to autoimport a recipe with the exact same title and the exact same source URL, warn the user there is a recipe with the exact same title and the exact same source URL.