title: Merge map of arrays with duplicate keys
body: <p>I have two maps of arrays. </p>
  
  <pre><code>Map&lt;String, List&lt;String&gt;&gt; map1 = new HashMap&lt;&gt;();
  Map&lt;String, List&lt;String&gt;&gt; map2 = new HashMap&lt;&gt;();
  </code></pre>
  
  <p>I want to merge them in one new map.<br>
  If a key exists in both maps, in that case, I should merge arrays. </p>
  
  <p>For example:</p>
  
  <pre><code>map1.put("k1", Arrays.asList("a0", "a1"));
  map1.put("k2", Arrays.asList("b0", "b1"));
  
  map2.put("k2", Arrays.asList("z1", "z2"));
  
  // Expected output is 
  Map 3: {k1=[a0, a1], k2=[b0, b1, z1, z2]}
  </code></pre>
  
  <p>I tried to do that with streams </p>
  
  <pre><code>Map&lt;String, List&lt;String&gt;&gt; map3 = Stream.of(map1, map2)
      .flatMap(map -&gt; map.entrySet().stream())
      .collect(Collectors.toMap(
          Map.Entry::getKey,
          e -&gt; e.getValue().stream().collect(Collectors.toList())
      ));
  </code></pre>
  
  <p>This work if there are no the same keys in maps. Otherwise, I get the exception  </p>
  
  <pre><code>Exception in thread "main" java.lang.IllegalStateException: Duplicate key k2 (attempted merging values [b0, b1] and [z1, z2])
      at java.base/java.util.stream.Collectors.duplicateKeyException(Collectors.java:133)
      at java.base/java.util.stream.Collectors.lambda$uniqKeysMapAccumulator$1(Collectors.java:180)
      at java.base/java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
      at java.base/java.util.HashMap$EntrySpliterator.forEachRemaining(HashMap.java:1751)
      at java.base/java.util.stream.ReferencePipeline$Head.forEach(ReferencePipeline.java:658)
      at java.base/java.util.stream.ReferencePipeline$7$1.accept(ReferencePipeline.java:274)
      at java.base/java.util.Spliterators$ArraySpliterator.forEachRemaining(Spliterators.java:948)
      at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:484)
      at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:474)
      at java.base/java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:913)
      at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
      at java.base/java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:578)
      at im.djm.Test.main(Test.java:25)
  </code></pre>
  
  <p>Is there a way to accomplish this task with streams?<br>
  Or I have to iterate throug maps? </p>