Index: applications/editors/josm/plugins/wikipedia/src/org/wikipedia/WikipediaApp.java
===================================================================
--- applications/editors/josm/plugins/wikipedia/src/org/wikipedia/WikipediaApp.java	(revision 33065)
+++ applications/editors/josm/plugins/wikipedia/src/org/wikipedia/WikipediaApp.java	(revision 33066)
@@ -18,4 +18,5 @@
 import java.util.Objects;
 import java.util.TreeMap;
+import java.util.TreeSet;
 import java.util.regex.Pattern;
 import java.util.stream.Collectors;
@@ -194,5 +195,6 @@
     public static Map<String, String> getWikidataForArticles(String wikipediaLang, List<String> articles) {
         if (articles.size() > 50) {
-            return partitionList(articles, 50).stream()
+            final List<String> withoutDuplicates = new ArrayList<>(new TreeSet<>(articles));
+            return partitionList(withoutDuplicates, 50).stream()
                     .flatMap(chunk -> getWikidataForArticles(wikipediaLang, chunk).entrySet().stream())
                     .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
Index: applications/editors/josm/plugins/wikipedia/test/unit/org/wikipedia/WikipediaAppTest.java
===================================================================
--- applications/editors/josm/plugins/wikipedia/test/unit/org/wikipedia/WikipediaAppTest.java	(revision 33065)
+++ applications/editors/josm/plugins/wikipedia/test/unit/org/wikipedia/WikipediaAppTest.java	(revision 33066)
@@ -8,9 +8,14 @@
 import org.wikipedia.data.WikipediaEntry;
 
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.List;
 import java.util.Locale;
 import java.util.Map;
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
+import java.util.stream.Stream;
 
 import static org.hamcrest.CoreMatchers.hasItem;
@@ -115,4 +120,11 @@
 
     @Test
+    public void testTicket13991() throws Exception {
+        final Map<String, String> map = WikipediaApp.getWikidataForArticles("en",
+                Stream.iterate("London", x -> x).limit(100).collect(Collectors.toList()));
+        assertThat(map, is(Collections.singletonMap("London", "Q84")));
+    }
+
+    @Test
     public void testGetLabelForWikidata() throws Exception {
         assertThat(WikipediaApp.getLabelForWikidata("Q1741", Locale.GERMAN), is("Wien"));