Duplicate OperationIds

[spaceraccoon]

Although GrapeSwagger::DocMethods::OperationId.build implements a mangling algorithm to generate an operation ID from a path and method, it can lead to duplicates such as /api/v4/projects/{id} and /api/v4/projects{id} both evaluating to the same operation ID, which is invalid. Although this may be an unusual path for an API, some organisations may have them as a legacy. There should be a check for duplicates to prevent this.