+---------+-------+
| baf18c4 | Item1 |
| c5e4623 | Item2 |
| bf1932f | Item3 |
+---------+-------+

+---------+-------+
| baf18c4 | Item1 |
| c5e4623 | Item2 |
| baf18c4 | Item3 |
| baf18c4 | Item4 |
+---------+-------+

+---------+-------+
| baf18c4 | Item1 |
| c5e4623 | Item2 |
| baf18c4 | Item3 |
| bf1932f | Item4 |
| c5e4623 | Item5 |
+---------+-------+

package tmp;

import static org.junit.Assert.assertEquals;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.stream.Collectors;

import org.junit.Test;

public class Snippet {
  private static String verifyItems(List<Item> items) {
    Map<String, List<Item>> map = new HashMap<>();
    for (Item item : items) {
      String key = item.getCode();
      List<Item> list;
      if (map.containsKey(key)) {
        list = map.get(key);
      }
      else {
        list = new ArrayList<>();
        map.put(key, list);
      }
      list.add(item);
    }
    boolean isFirst = true;
    StringBuilder sb = new StringBuilder();
    for (Entry<String, List<Item>> e : map.entrySet()) {
      if (e.getValue().size() > 1) {
        if (isFirst) {
          sb.append("Some of the items are sharing the same code: ");
        }
        else {
          sb.append(", ");
        }
        isFirst = false;
        sb.append("'" + e.getKey() + "' used by ");
        sb.append(e.getValue().stream().map(i -> i.getDescription())
            .collect(Collectors.joining(", ", "{", "}")));
      }
    }
    return sb.toString();
  }

  @Test
  public void testEmpty() throws Exception {
    assertEquals("", verifyItems(Collections.emptyList()));
  }

  @Test
  public void testOk() throws Exception {
    List<Item> items = Arrays.asList(
        new Item("baf18c4", "Item1"),
        new Item("c5e4623", "Item2"),
        new Item("bf1932f", "Item3"));
    assertEquals("", verifyItems(items));
  }

  @Test
  public void testDuplicate() throws Exception {
    List<Item> items = Arrays.asList(
        new Item("baf18c4", "Item1"),
        new Item("c5e4623", "Item2"),
        new Item("baf18c4", "Item3"),
        new Item("baf18c4", "Item4"));
    assertEquals("Some of the items are sharing the same code: 'baf18c4' used by {Item1, Item3, Item4}", verifyItems(items));
  }

  @Test
  public void testDuplicate2() throws Exception {
    List<Item> items = Arrays.asList(
        new Item("baf18c4", "Item1"),
        new Item("c5e4623", "Item2"),
        new Item("baf18c4", "Item3"),
        new Item("bf1932f", "Item4"),
        new Item("c5e4623", "Item5"));
    assertEquals("Some of the items are sharing the same code: 'baf18c4' used by {Item1, Item3}, 'c5e4623' used by {Item2, Item5}", verifyItems(items));
  }

  public static class Item {
    private String code;
    private String description;

    public Item(String code, String description) {
      super();
      this.code = code;
      this.description = description;
    }

    public String getCode() {
      return code;
    }

    public String getDescription() {
      return description;
    }
  }
}