Another option to select the itens that are not duplicated in the stream

BestApproach.java

package com.moogu.restore;

import java.util.*;
import java.util.stream.Stream;

public class BestApproach {

	public static void main(String[] args) {
		List<String> letters = Arrays.asList("a", "b", "c", "d", "e", "f", "g", "f", "e", "c", "b");
		BestApproach main = new BestApproach();
		System.out.println(main.processStream(letters.stream()));
	}

	private List<String> processStream(Stream<String> stream) {
		final Map<String, Integer> content = new HashMap<>();
		final List<String> result = new ArrayList<>();

		// add content to hash
		stream.forEach(x -> addToHash(x, content));

		// filter
		for (Map.Entry<String, Integer> entry : content.entrySet()) {
			if (entry.getValue() == 1) {
				result.add(entry.getKey());
			}
		}

		return result;
	}

	private void addToHash(String x, Map<String, Integer> content) {
		if (content.containsKey(x)) {
			Integer actualValue = content.get(x);
			content.put(x, ++actualValue);
		} else {
			content.put(x, 1);
		}
	}
}