Throw an exception if the same name is written to JSON more than once

Update `JsonValueWriter` to track written names and throw an exception
if there is a duplicate.

Closes gh-43041

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/json/JsonValueWriter.java

@@ -21,7 +21,9 @@ import java.io.UncheckedIOException;
 import java.util.ArrayDeque;
 import java.util.Arrays;
 import java.util.Deque;
+import java.util.HashSet;
 import java.util.Map;
+import java.util.Set;
 import java.util.function.BiConsumer;
 import java.util.function.Consumer;
 import java.util.function.Predicate;
@@ -223,6 +225,8 @@ class JsonValueWriter {
 			ActiveSeries activeSeries = this.activeSeries.peek();
 			Assert.notNull(activeSeries, "No series has been started");
 			activeSeries.incrementIndexAndAddCommaIfRequired();
+			Assert.state(activeSeries.addName(processedName),
+					() -> "The name '" + processedName + "' has already been written");
 			writeString(processedName);
 			append(":");
 			write(value);
@@ -359,10 +363,16 @@ class JsonValueWriter {
 
 		private int index;
 
+		private Set<String> names = new HashSet<>();
+
 		private ActiveSeries(Series series) {
 			this.series = series;
 		}
 
+		boolean addName(String processedName) {
+			return this.names.add(processedName);
+		}
+
 		MemberPath updatePath(MemberPath path) {
 			return (this.series != Series.ARRAY) ? path : path.child(this.index);
 		}

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/json/JsonValueWriterTests.java

@@ -29,6 +29,7 @@ import org.springframework.boot.json.JsonValueWriter.Series;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
+import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
 
 /**
  * Tests for {@link JsonValueWriter} .
@@ -193,6 +194,16 @@ class JsonValueWriterTests {
 				{"a":"A","b":"B"}""");
 	}
 
+	@Test
+	void writePairsWhenDuplicateThrowsException() {
+		assertThatIllegalStateException().isThrownBy(() -> doWrite((valueWriter) -> {
+			valueWriter.start(Series.OBJECT);
+			valueWriter.writePairs(Map.of("a", "A")::forEach);
+			valueWriter.writePairs(Map.of("a", "B")::forEach);
+			valueWriter.end(Series.OBJECT);
+		})).withMessage("The name 'a' has already been written");
+	}
+
 	@Test
 	void writeArray() {
 		List<String> list = List.of("a", "b", "c");