Allow users to add single duplicate issues and epics to the issue and epic widgets without triggering an error

If a user adds a duplicate issue or epic (here is an issue), an error appears.

Even once a user deletes the offending issue and adds a new one, the error is still present and the add button looks disabled (even though it is not), which is a confusing state to be in.

When a user adds a duplicate issue AND a new issue;

• There is no error state
• The new issue is added
• The duplicate issue is not added but appears in the list as it is already there

I propose we stop triggering an error when a user adds a duplicate issue. I think we should just allow them to add it and assume they didn't see it within the list already (and track this to investigate why this is the case). This is actually less work on behalf the user and interrupts their workflow less than our current state. In the future perhaps we could explore real time validation states so that we catch duplicate issues before they are added and alert the user.