Is ConcurrentDictionary.GetOrAdd() guaranteed to invoke valueFactoryMethod only once per key?

As others have already pointed out, valueFactory may be invoked more than once. There is a common solution that mitigates this issue - have your valueFactory return a Lazy<T> instance. Although it's possible that multiple lazy instances will be created, the actual T value will only be created when you access Lazy<T>.Value property.

Specifically:

// Lazy instance may be created multiple times, but only one will actually be used.
// GetObjectFromRemoteServer will not be called here.
var lazyObject = dict.GetOrAdd("key", key => new Lazy<MyObject>(() => GetObjectFromRemoteServer()));

// Only here GetObjectFromRemoteServer() will be called.
// The next calls will not go to the server
var myObject = lazyObject.Value;

This method is further explained in Reed Copsey's blog post

Is value factory invoked only once per key?

No, it isn't. The docs say:

If you call GetOrAdd simultaneously on different threads, valueFactory may be invoked multiple times, but its key/value pair might not be added to the dictionary for every call.

Let's take a look at the source code of GetOrAdd:

public TValue GetOrAdd(TKey key, Func<TKey, TValue> valueFactory)
{
    if (key == null) throw new ArgumentNullException("key");
    if (valueFactory == null) throw new ArgumentNullException("valueFactory");

    TValue resultingValue;
    if (TryGetValue(key, out resultingValue))
    {
        return resultingValue;
    }
    TryAddInternal(key, valueFactory(key), false, true, out resultingValue);
    return resultingValue;
}

Unfortunately, in this case, it's clear nothing guarantees that valueFactory won't be called more than once, if two GetOrAdd invocations happen to run in parallel.