No notification when creating a label which already exists

Related to: #26945 (closed)

When a user tried to create a label which already existed on the new/edit issue page. There was no notification to inform him that the label already existed, therefore it wasn’t clear to him what was happening. [Clip 1] (https://youtu.be/UF_zfCYgukw?t=11m22s).

I think we need a message here to let the user know that the label already exists.

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Proposal