Opening a file removes duplicate keywords and triggers lib changed

src/main/java/org/jabref/model/entry/BibEntry.java

@@ -773,8 +773,20 @@ public KeywordList getResolvedKeywords(Character delimiter, BibDatabase database
 
     public Optional<FieldChange> removeKeywords(KeywordList keywordsToRemove, Character keywordDelimiter) {
         KeywordList keywordList = getKeywords(keywordDelimiter);
+
+        // We need to fix "file has changed on disk" for duplicate keywords
+        // The input of the set may contain duplicate keywords (which are present as single keyword in the set)
+        // Even if no "keywordsToRemove" is contained, the method "putKeywords" will return a change, because the duplicate keywords will have been removed
+        int oldSize = keywordList.size();
         keywordList.removeAll(keywordsToRemove);
-        return putKeywords(keywordList, keywordDelimiter);
+        // claim 1: The size of a set is the same, if no element is removed
+        // claim 2: The size of a set is different if an element was removed
+        // With claim 1, we can assume no change if there is no change on the size
+        if (oldSize == keywordList.size()) {
+            return Optional.empty();
+        } else {
+            return putKeywords(keywordList, keywordDelimiter);
+        }
     }
 
     public Optional<FieldChange> replaceKeywords(KeywordList keywordsToReplace,

src/test/java/org/jabref/logic/exporter/BibtexDatabaseWriterTest.java

@@ -157,6 +157,37 @@ void writeEntry() throws Exception {
                 stringWriter.toString());
     }
 
+    @Test
+    void writeEntryWithDuplicateKeywords() throws Exception {
+        BibEntry entry = new BibEntry();
+        entry.setType(StandardEntryType.Article);
+        entry.setField(StandardField.KEYWORDS, "asdf,asdf,asdf");
+        database.insertEntry(entry);
+
+        databaseWriter.savePartOfDatabase(bibtexContext, Collections.singletonList(entry));
+
+        assertEquals("@Article{," + OS.NEWLINE
+                     + "  keywords = {asdf,asdf,asdf}," + OS.NEWLINE
+                     + "}" + OS.NEWLINE,
+                stringWriter.toString());
+    }
+
+    @Test
+    void putKeyWordsRemovesDuplicateKeywordsIsVisibleDuringWrite() throws Exception {
+        BibEntry entry = new BibEntry();
+        entry.setType(StandardEntryType.Article);
+        entry.putKeywords(List.of("asdf", "asdf", "asdf"), ',');
+
+        database.insertEntry(entry);
+
+        databaseWriter.savePartOfDatabase(bibtexContext, Collections.singletonList(entry));
+
+        assertEquals("@Article{," + OS.NEWLINE
+                     + "  keywords = {asdf}," + OS.NEWLINE
+                     + "}" + OS.NEWLINE,
+                stringWriter.toString());
+    }
+
     @Test
     void writeEncodingAndEntry() throws Exception {
         BibEntry entry = new BibEntry();

src/test/java/org/jabref/logic/importer/fileformat/BibtexParserTest.java

@@ -1923,4 +1923,39 @@ void bibTeXConstantAprilIsParsedAsStringMonthAprilWhenReadingTheField() throws P
 
         assertEquals(Optional.of("#apr#"), result.get().getField(StandardField.MONTH));
     }
+
+    @Test
+    void parseDuplicateKeywordsWithOnlyOneEntry() throws ParseException {
+        Optional<BibEntry> result = parser.parseSingleEntry("@Article{,\n"
+            + "Keywords={asdf,asdf,asdf},\n"
+            + "}\n"
+            + "");
+
+        BibEntry expectedEntry = new BibEntry(StandardEntryType.Article)
+            .withField(StandardField.KEYWORDS, "asdf,asdf,asdf");
+
+        assertEquals(Optional.of(expectedEntry), result);
+    }
+
+    @Test
+    void parseDuplicateKeywordsWithTwoEntries() throws Exception {
+        BibEntry expectedEntryFirst = new BibEntry(StandardEntryType.Article)
+            .withField(StandardField.KEYWORDS, "bbb")
+            .withCitationKey("Test2017");
+
+        BibEntry expectedEntrySecond = new BibEntry(StandardEntryType.Article)
+            .withField(StandardField.KEYWORDS, "asdf,asdf,asdf");
+
+        String entries = """
+            @Article{Test2017,
+              keywords = {bbb},
+            }
+
+            @Article{,
+              keywords = {asdf,asdf,asdf},
+            },
+            """;
+        ParserResult result = parser.parse(new StringReader(entries));
+        assertEquals(List.of(expectedEntryFirst, expectedEntrySecond), result.getDatabase().getEntries());
+    }
 }

src/test/java/org/jabref/migrations/SpecialFieldsToSeparateFieldsTest.java

@@ -39,6 +39,18 @@ public void noKewordToMigrate() {
         assertEquals(expected, entry);
     }
 
+    @Test
+    public void noKeywordToMigrateButDuplicateKeywords() {
+        BibEntry entry = new BibEntry().withField(StandardField.AUTHOR, "JabRef")
+                                       .withField(StandardField.KEYWORDS, "asdf, asdf, asdf");
+        BibEntry expected = new BibEntry().withField(StandardField.AUTHOR, "JabRef")
+                                          .withField(StandardField.KEYWORDS, "asdf, asdf, asdf");
+
+        new SpecialFieldsToSeparateFields(',').performMigration(new ParserResult(List.of(entry)));
+
+        assertEquals(expected, entry);
+    }
+
     @Test
     public void migrateMultipleSpecialFields() {
         BibEntry entry = new BibEntry().withField(StandardField.AUTHOR, "JabRef")

src/test/java/org/jabref/model/database/BibDatabaseTest.java

@@ -30,7 +30,7 @@
 class BibDatabaseTest {
 
     private BibDatabase database;
-    private BibtexString bibtexString = new BibtexString("DSP", "Digital Signal Processing");
+    private final BibtexString bibtexString = new BibtexString("DSP", "Digital Signal Processing");
 
     @BeforeEach
     void setUp() {