Refine MetadataCollector logic

Update `MetadataCollector` merge logic so that previous items are no
longer added if the current round contains a property of the same name.

Fixes gh-23916

spring-boot-project/spring-boot-tools/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/MetadataCollector.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -98,7 +98,7 @@ public class MetadataCollector {
 			List<ItemMetadata> items = this.previousMetadata.getItems();
 			for (ItemMetadata item : items) {
 				if (shouldBeMerged(item)) {
-					metadata.add(item);
+					metadata.addIfMissing(item);
 				}
 			}
 		}

spring-boot-project/spring-boot-tools/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/metadata/ConfigurationMetadata.java

@@ -62,7 +62,15 @@ public class ConfigurationMetadata {
 	 * @param itemMetadata the meta-data to add
 	 */
 	public void add(ItemMetadata itemMetadata) {
-		add(this.items, itemMetadata.getName(), itemMetadata);
+		add(this.items, itemMetadata.getName(), itemMetadata, false);
+	}
+
+	/**
+	 * Add item meta-data if it's not already present.
+	 * @param itemMetadata the meta-data to add
+	 */
+	public void addIfMissing(ItemMetadata itemMetadata) {
+		add(this.items, itemMetadata.getName(), itemMetadata, true);
 	}
 
 	/**
@@ -70,7 +78,7 @@ public class ConfigurationMetadata {
 	 * @param itemHint the item hint to add
 	 */
 	public void add(ItemHint itemHint) {
-		add(this.hints, itemHint.getName(), itemHint);
+		add(this.hints, itemHint.getName(), itemHint, false);
 	}
 
 	/**
@@ -131,13 +139,15 @@ public class ConfigurationMetadata {
 			}
 		}
 		else {
-			add(this.items, metadata.getName(), metadata);
+			add(this.items, metadata.getName(), metadata, false);
 		}
 	}
 
-	private <K, V> void add(Map<K, List<V>> map, K key, V value) {
+	private <K, V> void add(Map<K, List<V>> map, K key, V value, boolean ifMissing) {
 		List<V> values = map.computeIfAbsent(key, (k) -> new ArrayList<>());
-		values.add(value);
+		if (!ifMissing || values.isEmpty()) {
+			values.add(value);
+		}
 	}
 
 	private ItemMetadata findMatchingItemMetadata(ItemMetadata metadata) {

spring-boot-project/spring-boot-tools/spring-boot-configuration-processor/src/test/java/org/springframework/boot/configurationprocessor/metadata/Metadata.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -25,6 +25,7 @@ import org.assertj.core.api.Condition;
 import org.hamcrest.collection.IsMapContaining;
 
 import org.springframework.boot.configurationprocessor.metadata.ItemMetadata.ItemType;
+import org.springframework.util.Assert;
 import org.springframework.util.ObjectUtils;
 
 /**
@@ -131,7 +132,7 @@ public final class Metadata {
 
 		@Override
 		public boolean matches(ConfigurationMetadata value) {
-			ItemMetadata itemMetadata = getFirstItemWithName(value, this.name);
+			ItemMetadata itemMetadata = getItemWithName(value, this.name);
 			if (itemMetadata == null) {
 				return false;
 			}
@@ -207,13 +208,15 @@ public final class Metadata {
 					this.description, this.defaultValue, null);
 		}
 
-		private ItemMetadata getFirstItemWithName(ConfigurationMetadata metadata, String name) {
+		private ItemMetadata getItemWithName(ConfigurationMetadata metadata, String name) {
+			ItemMetadata result = null;
 			for (ItemMetadata item : metadata.getItems()) {
 				if (item.isOfItemType(this.itemType) && name.equals(item.getName())) {
-					return item;
+					Assert.state(result == null, () -> "Duplicate item found for " + name);
+					result = item;
 				}
 			}
-			return null;
+			return result;
 		}
 
 	}