diff --git a/cdc-service/src/main/java/com/xtrmetl/cdc/spi/CdcSourceFactory.java b/cdc-service/src/main/java/com/xtrmetl/cdc/spi/CdcSourceFactory.java
index d389b166..d9be8af1 100644
--- a/cdc-service/src/main/java/com/xtrmetl/cdc/spi/CdcSourceFactory.java
+++ b/cdc-service/src/main/java/com/xtrmetl/cdc/spi/CdcSourceFactory.java
@@ -3,10 +3,12 @@
 import org.springframework.stereotype.Component;
 
 import java.util.ArrayList;
+import java.util.HashSet;
 import java.util.List;
 import java.util.Locale;
 import java.util.Map;
 import java.util.Optional;
+import java.util.Set;
 
 /**
  * Resolves configured CDC source type ids to {@link CdcSourceConnector} instances.
@@ -32,15 +34,23 @@ public Optional<CdcSourceConnector> resolve(String type) {
     }
 
     /**
-     * Validate a multi-source config list. Does not start engines — live capture
-     * remains single-source in {@code CdcService}.
+     * Validates and describes a multi-source configuration list without starting engines.
+     * Live capture remains single-source in {@code CdcService}.
+     *
+     * @param specs configured source entries; {@code null} is treated as an empty list
+     * @return one descriptive row for each configured source
+     * @throws IllegalArgumentException when two entries declare the same source id
      */
     public List<Map<String, Object>> describeConfigured(List<SourceSpec> specs) {
         List<Map<String, Object>> out = new ArrayList<>();
         if (specs == null) {
             return out;
         }
+        Set<String> sourceIds = new HashSet<>();
         for (SourceSpec spec : specs) {
+            if (!sourceIds.add(spec.id())) {
+                throw new IllegalArgumentException("duplicate source id: " + spec.id());
+            }
             Map<String, Object> row = new java.util.LinkedHashMap<>();
             row.put("id", spec.id());
             row.put("type", spec.type());
diff --git a/cdc-service/src/test/java/com/xtrmetl/cdc/spi/CdcSourceFactoryIdentityContractTest.java b/cdc-service/src/test/java/com/xtrmetl/cdc/spi/CdcSourceFactoryIdentityContractTest.java
new file mode 100644
index 00000000..6bd004b7
--- /dev/null
+++ b/cdc-service/src/test/java/com/xtrmetl/cdc/spi/CdcSourceFactoryIdentityContractTest.java
@@ -0,0 +1,44 @@
+package com.xtrmetl.cdc.spi;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+/**
+ * Protects configured CDC source identity from ambiguous duplicate declarations.
+ */
+class CdcSourceFactoryIdentityContractTest {
+
+    @Test
+    void duplicateConfiguredSourceIdsFailClosed() {
+        CdcSourceFactory factory = factory();
+
+        IllegalArgumentException failure = assertThrows(
+                IllegalArgumentException.class,
+                () -> factory.describeConfigured(List.of(
+                        new CdcSourceFactory.SourceSpec("pg-main", "postgres-debezium", true),
+                        new CdcSourceFactory.SourceSpec("pg-main", "postgres-debezium", false)
+                ))
+        );
+
+        assertTrue(failure.getMessage().contains("pg-main"));
+    }
+
+    @Test
+    void repeatedSourceTypeWithDistinctIdsRemainsValid() {
+        CdcSourceFactory factory = factory();
+
+        assertDoesNotThrow(() -> factory.describeConfigured(List.of(
+                new CdcSourceFactory.SourceSpec("pg-primary", "postgres-debezium", true),
+                new CdcSourceFactory.SourceSpec("pg-secondary", "postgres-debezium", false)
+        )));
+    }
+
+    private static CdcSourceFactory factory() {
+        return new CdcSourceFactory(new CdcSourceRegistry(List.of(new PostgresDebeziumCdcSource())));
+    }
+}