rename fails when previousNames matches more than one label

[Xiphe]

Given I have two labels "foo" and "bar"
And I want to rename them with

{
  "previousNames": [
    "foo",
    "bar"
  ],
  "name": "Foo",
  "color": "..."
}

Then I expect to have one label "Foo" after labels have been synced
But I see an error

[ insert error here ]

cc @KlausTrainer

[Xiphe]

this is partially a feature since merging of two labels to one is (intentionally) not possible right now