notification can override provider's preventDup

[williammetcalf]

Currently if a provider has preventDuplicates={true}, then duplicates will be prevented even if a individual notification has preventDuplicates explicitly set to false.

This PR changes that behavior, so duplicates will never be prevented for some notification if it has preventDuplicates: false, regardless of the provider settings

[iamhosseindhv]

Thanks for the PR @williammetcalf . Current implementation is intentional. Prevent duplicate is something that's configured on a Provider level and not individual snackbar.

Is there a specific use case you have where you need it to be overridden?

[williammetcalf]

In the docs preventDuplicate is listed in the "Mutual Props" section.

My use case is in our app we want most of our notifications to have duplicates prevented, but every now and then we want a type of notification which can be duplicated. It would be easier to prevent duplicates by default and allow them explicitly in the few places we need, rather than the other way around.

[iamhosseindhv]

I see @williammetcalf. Thanks for your contribution.