Duplicate custom field in alerting is not accepted

## Expected Behavior
I expect a duplicate field to be flagged as a validation error.

## Current Behavior
As soon as the new field name is equal to an existing one, the DOM element disappears.

## Possible Solution
Very likely the two components use the same react key, so the DOM element disappears.

## Steps to Reproduce (for bugs)
1. Create an alert definition, enter one field with any name.
2. Create a second custom field, try to enter the same name.

## Context

## Your Environment

* Graylog Version: 3.1 master, alert-definitions-ui