Wildcard field synthetic source doesn't look like it handles duplicate entries.

**Build scan:**
https://gradle-enterprise.elastic.co/s/bhxzqzxu5vnw6/tests/:x-pack:plugin:wildcard:test/org.elasticsearch.xpack.wildcard.mapper.WildcardFieldMapperTests/testSyntheticSourceMany

**Reproduction line:**
`./gradlew ':x-pack:plugin:wildcard:test' --tests "org.elasticsearch.xpack.wildcard.mapper.WildcardFieldMapperTests.testSyntheticSourceMany" -Dtests.seed=448AC91905D79C3A -Dtests.locale=es-GT -Dtests.timezone=Asia/Nicosia -Druntime.java=17`

**Applicable branches:**
main

**Reproduces locally?:**
Yes

**Failure history:**
https://gradle-enterprise.elastic.co/scans/tests?tests.container=org.elasticsearch.xpack.wildcard.mapper.WildcardFieldMapperTests&tests.test=testSyntheticSourceMany

**Failure excerpt:**
```
java.lang.AssertionError: doc 2
Expected: "{\"field\":[\"EX\",\"EX\",\"HiKnr\",\"elGnt\"]}"
     but: was "{\"field\":[\"EX\",\"HiKnr\",\"elGnt\"]}"

  at __randomizedtesting.SeedInfo.seed([448AC91905D79C3A:5E6D240C8864041E]:0)
  at org.hamcrest.MatcherAssert.assertThat(MatcherAssert.java:18)
  at org.junit.Assert.assertThat(Assert.java:956)
  at org.elasticsearch.index.mapper.MapperTestCase.testSyntheticSourceMany(MapperTestCase.java:1072)
  at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(NativeMethodAccessorImpl.java:-2)
  at jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
  at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
  at java.lang.reflect.Method.invoke(Method.java:568)
  at com.carrotsearch.randomizedtesting.RandomizedRunner.invoke(RandomizedRunner.java:1758)
  at com.carrotsearch.randomizedtesting.RandomizedRunner$8.evaluate(RandomizedRunner.java:946)
  at com.carrotsearch.randomizedtesting.RandomizedRunner$9.evaluate(RandomizedRunner.java:982)
  at com.carrotsearch.randomizedtesting.RandomizedRunner$10.evaluate(RandomizedRunner.java:996)
  at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
  at org.apache.lucene.tests.util.TestRuleSetupTeardownChained$1.evaluate(TestRuleSetupTeardownChained.java:44)
  at org.apache.lucene.tests.util.AbstractBeforeAfterRule$1.evaluate(AbstractBeforeAfterRule.java:43)
  at org.apache.lucene.tests.util.TestRuleThreadAndTestName$1.evaluate(TestRuleThreadAndTestName.java:45)
  at org.apache.lucene.tests.util.TestRuleIgnoreAfterMaxFailures$1.evaluate(TestRuleIgnoreAfterMaxFailures.java:60)
  at org.apache.lucene.tests.util.TestRuleMarkFailure$1.evaluate(TestRuleMarkFailure.java:44)
  at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
  at com.carrotsearch.randomizedtesting.ThreadLeakControl$StatementRunner.run(ThreadLeakControl.java:390)
  at com.carrotsearch.randomizedtesting.ThreadLeakControl.forkTimeoutingTask(ThreadLeakControl.java:843)
  at com.carrotsearch.randomizedtesting.ThreadLeakControl$3.evaluate(ThreadLeakControl.java:490)
  at com.carrotsearch.randomizedtesting.RandomizedRunner.runSingleTest(RandomizedRunner.java:955)
  at com.carrotsearch.randomizedtesting.RandomizedRunner$5.evaluate(RandomizedRunner.java:840)
  at com.carrotsearch.randomizedtesting.RandomizedRunner$6.evaluate(RandomizedRunner.java:891)
  at com.carrotsearch.randomizedtesting.RandomizedRunner$7.evaluate(RandomizedRunner.java:902)
  at org.apache.lucene.tests.util.AbstractBeforeAfterRule$1.evaluate(AbstractBeforeAfterRule.java:43)
  at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
  at org.apache.lucene.tests.util.TestRuleStoreClassName$1.evaluate(TestRuleStoreClassName.java:38)
  at com.carrotsearch.randomizedtesting.rules.NoShadowingOrOverridesOnMethodsRule$1.evaluate(NoShadowingOrOverridesOnMethodsRule.java:40)
  at com.carrotsearch.randomizedtesting.rules.NoShadowingOrOverridesOnMethodsRule$1.evaluate(NoShadowingOrOverridesOnMethodsRule.java:40)
  at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
  at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
  at org.apache.lucene.tests.util.TestRuleAssertionsRequired$1.evaluate(TestRuleAssertionsRequired.java:53)
  at org.apache.lucene.tests.util.AbstractBeforeAfterRule$1.evaluate(AbstractBeforeAfterRule.java:43)
  at org.apache.lucene.tests.util.TestRuleMarkFailure$1.evaluate(TestRuleMarkFailure.java:44)
  at org.apache.lucene.tests.util.TestRuleIgnoreAfterMaxFailures$1.evaluate(TestRuleIgnoreAfterMaxFailures.java:60)
  at org.apache.lucene.tests.util.TestRuleIgnoreTestSuites$1.evaluate(TestRuleIgnoreTestSuites.java:47)
  at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
  at com.carrotsearch.randomizedtesting.ThreadLeakControl$StatementRunner.run(ThreadLeakControl.java:390)
  at com.carrotsearch.randomizedtesting.ThreadLeakControl.lambda$forkTimeoutingTask$0(ThreadLeakControl.java:850)
  at java.lang.Thread.run(Thread.java:833)

```
x-pack/plugin/wildcard/src/test/java/org/elasticsearch/xpack/wildcard/mapper/WildcardFieldMapperTests.java
@Override
    protected void minimalMapping(XContentBuilder b) throws IOException {
        b.field("type", "wildcard");
    }
    @Override
    protected Object getSampleValueForDocument() {
        return "test";
    }
    @Override
    protected void registerParameters(ParameterChecker checker) throws IOException {
        checker.registerConflictCheck("null_value", b -> b.field("null_value", "foo"));
        checker.registerUpdateCheck(b -> b.field("ignore_above", 256), m -> assertEquals(256, ((WildcardFieldMapper) m).ignoreAbove()));
    }
    static class FuzzyTest {
        String pattern;
        int prefixLength;
        Fuzziness fuzziness;
        String expectedPrefixQuery;
        int expectedMinShouldMatch;
        String ngrams;
        FuzzyTest(
            String pattern,
            int prefixLength,
            Fuzziness fuzziness,
            String expectedPrefixQuery,
            int expectedMinShouldMatch,
            String ngrams
        ) {
            super();
            this.pattern = pattern;