diff --git a/spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/properties/source/ConfigurationPropertyName.java b/spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/properties/source/ConfigurationPropertyName.java
index f4b31f8259..6c798ad833 100644
--- a/spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/properties/source/ConfigurationPropertyName.java
+++ b/spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/properties/source/ConfigurationPropertyName.java
@@ -347,7 +347,7 @@ public final class ConfigurationPropertyName
 			}
 		}
 		while (i2 < l2) {
-			char ch2 = e2.charAt(i, i2++);
+			char ch2 = Character.toLowerCase(e2.charAt(i, i2++));
 			if (indexed2 || ElementsParser.isAlphaNumeric(ch2)) {
 				return false;
 			}
diff --git a/spring-boot-project/spring-boot/src/test/java/org/springframework/boot/context/properties/source/ConfigurationPropertyNameTests.java b/spring-boot-project/spring-boot/src/test/java/org/springframework/boot/context/properties/source/ConfigurationPropertyNameTests.java
index 43bfa6de13..4739f8f5c3 100644
--- a/spring-boot-project/spring-boot/src/test/java/org/springframework/boot/context/properties/source/ConfigurationPropertyNameTests.java
+++ b/spring-boot-project/spring-boot/src/test/java/org/springframework/boot/context/properties/source/ConfigurationPropertyNameTests.java
@@ -609,6 +609,26 @@ public class ConfigurationPropertyNameTests {
 		assertThat(n1).isNotEqualTo(n2);
 	}
 
+	@Test
+	public void equalsWhenStartsWithOfAdaptedName() {
+		// gh-15152
+		ConfigurationPropertyName n1 = ConfigurationPropertyName
+				.adapt("example.mymap.ALPHA", '.');
+		ConfigurationPropertyName n2 = ConfigurationPropertyName
+				.adapt("example.mymap.ALPHA_BRAVO", '.');
+		assertThat(n1).isNotEqualTo(n2);
+	}
+
+	@Test
+	public void equalsWhenStartsWithOfAdaptedNameOfIllegalChars() {
+		// gh-15152
+		ConfigurationPropertyName n1 = ConfigurationPropertyName
+				.adapt("example.mymap.ALPH!", '.');
+		ConfigurationPropertyName n2 = ConfigurationPropertyName
+				.adapt("example.mymap.ALPHA!BRAVO", '.');
+		assertThat(n1).isNotEqualTo(n2);
+	}
+
 	@Test
 	public void isValidWhenValidShouldReturnTrue() {
 		assertThat(ConfigurationPropertyName.isValid("")).isTrue();