diff --git a/spring-modulith-core/src/main/java/org/springframework/modulith/core/ApplicationModules.java b/spring-modulith-core/src/main/java/org/springframework/modulith/core/ApplicationModules.java
index 4b4d626c4..037b6c05a 100644
--- a/spring-modulith-core/src/main/java/org/springframework/modulith/core/ApplicationModules.java
+++ b/spring-modulith-core/src/main/java/org/springframework/modulith/core/ApplicationModules.java
@@ -143,18 +143,44 @@ private ApplicationModules(ModulithMetadata metadata, Collection<String> package
 				.distinct()
 				.collect(Collectors.toUnmodifiableSet());
 
-		this.modules = sources.stream() //
-				.map(it -> {
+		var exclusions = sources.stream()
+				.map(ApplicationModuleSource::getModuleBasePackage)
+				.toList();
+
+		var modules = sources.stream() //
+				.map(it -> new ApplicationModule(it, new JavaPackages(exclusions)))
+				.toList();
+
+		var duplicates = modules.stream()
+				.collect(groupingBy(ApplicationModule::getIdentifier))
+				.entrySet().stream()
+				.filter(it -> it.getValue().size() > 1)
+				.toList();
+
+		if (!duplicates.isEmpty()) {
 
-					var exclusions = sources.stream()
-							.map(ApplicationModuleSource::getModuleBasePackage)
-							.toList();
+			var message = duplicates.stream()
+					.map(it -> {
 
-					return new ApplicationModule(it, new JavaPackages(exclusions));
+						var basePackages = it.getValue().stream()
+								.map(module -> "'" + module.getBasePackage().getName() + "'")
+								.sorted()
+								.collect(joining(" and "));
 
-				})
+						return ("Application module identifier '%s' is not unique. The modules with the base packages %s use it."
+								+ " Declare an explicit identifier with @ApplicationModule(id = …), or set @Modulithic(useFullyQualifiedModuleNames = true).")
+										.formatted(it.getKey(), basePackages);
+
+					})
+					.collect(joining("\n"));
+
+			throw new IllegalStateException(message);
+		}
+
+		this.modules = modules.stream()
 				.collect(toMap(ApplicationModule::getIdentifier, Function.identity()));
 
+
 		this.rootPackages = packagesToImport.stream() //
 				.distinct()
 				.map(it -> JavaPackage.of(classes, it).toSingle()) //
diff --git a/spring-modulith-core/src/test/java/org/springframework/modulith/core/ApplicationModulesUnitTests.java b/spring-modulith-core/src/test/java/org/springframework/modulith/core/ApplicationModulesUnitTests.java
index 85f4c4ac9..bc6f05e41 100644
--- a/spring-modulith-core/src/test/java/org/springframework/modulith/core/ApplicationModulesUnitTests.java
+++ b/spring-modulith-core/src/test/java/org/springframework/modulith/core/ApplicationModulesUnitTests.java
@@ -89,4 +89,12 @@ void findsTypeBySimpleName() {
 			assertThat(it.contains("RootType")).isTrue();
 		});
 	}
+
+	@Test // GH-1964
+	void reportsDuplicateModuleIdentifiersWithDescriptiveMessage() {
+
+		assertThatIllegalStateException()
+				.isThrownBy(() -> TestUtils.of(reproducers.gh1964.app.Application.class))
+				.withMessage("Application module identifier 'billing' is not unique. The modules with the base packages 'reproducers.gh1964.first.billing' and 'reproducers.gh1964.second.billing' use it. Declare an explicit identifier with @ApplicationModule(id = …), or set @Modulithic(useFullyQualifiedModuleNames = true).");
+	}
 }
diff --git a/spring-modulith-core/src/test/java/reproducers/gh1964/Application.java b/spring-modulith-core/src/test/java/reproducers/gh1964/Application.java
new file mode 100644
index 000000000..0f8a0d55e
--- /dev/null
+++ b/spring-modulith-core/src/test/java/reproducers/gh1964/Application.java
@@ -0,0 +1,21 @@
+/*
+ * Copyright 2026 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package reproducers.gh1964;
+
+import org.springframework.modulith.Modulithic;
+
+@Modulithic(additionalPackages = { "reproducers.gh1964.first", "reproducers.gh1964.second" })
+public class Application {}
diff --git a/spring-modulith-core/src/test/java/reproducers/gh1964/app/Application.java b/spring-modulith-core/src/test/java/reproducers/gh1964/app/Application.java
new file mode 100644
index 000000000..4ebf6e115
--- /dev/null
+++ b/spring-modulith-core/src/test/java/reproducers/gh1964/app/Application.java
@@ -0,0 +1,21 @@
+/*
+ * Copyright 2026 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package reproducers.gh1964.app;
+
+import org.springframework.modulith.Modulithic;
+
+@Modulithic(additionalPackages = { "reproducers.gh1964.first", "reproducers.gh1964.second" })
+public class Application {}
diff --git a/spring-modulith-core/src/test/java/reproducers/gh1964/first/billing/FirstBilling.java b/spring-modulith-core/src/test/java/reproducers/gh1964/first/billing/FirstBilling.java
new file mode 100644
index 000000000..6300b212a
--- /dev/null
+++ b/spring-modulith-core/src/test/java/reproducers/gh1964/first/billing/FirstBilling.java
@@ -0,0 +1,18 @@
+/*
+ * Copyright 2026 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package reproducers.gh1964.first.billing;
+
+public class FirstBilling {}
diff --git a/spring-modulith-core/src/test/java/reproducers/gh1964/second/billing/SecondBilling.java b/spring-modulith-core/src/test/java/reproducers/gh1964/second/billing/SecondBilling.java
new file mode 100644
index 000000000..41a8199c8
--- /dev/null
+++ b/spring-modulith-core/src/test/java/reproducers/gh1964/second/billing/SecondBilling.java
@@ -0,0 +1,18 @@
+/*
+ * Copyright 2026 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package reproducers.gh1964.second.billing;
+
+public class SecondBilling {}