Github user robertdale commented on the issue:
https://github.com/apache/tinkerpop/pull/745
The race condition is in `put()` and it's used elsewhere. It would make
more sense to fix `put()` to be thread-safe. Then we can keep
`parallelStream()`
```
Map<Object, Set<T>> keyMap = this.index.get(key);
if (keyMap == null) {
keyMap = this.index.putIfAbsent(key, new ConcurrentHashMap<>());
}
Set<T> objects = keyMap.get(value);
if (null == objects) {
objects = keyMap.putIfAbsent(value,
ConcurrentHashMap.newKeySet());
}
objects.add(element);
```
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