diff --git a/formula/src/test/java/com/instacart/formula/FormulaRuntimeTest.kt b/formula/src/test/java/com/instacart/formula/FormulaRuntimeTest.kt
index d8f52823..2920f3ce 100644
--- a/formula/src/test/java/com/instacart/formula/FormulaRuntimeTest.kt
+++ b/formula/src/test/java/com/instacart/formula/FormulaRuntimeTest.kt
@@ -7,6 +7,7 @@ import com.instacart.formula.internal.ClearPluginsRule
 import com.instacart.formula.internal.TestInspector
 import com.instacart.formula.internal.Try
 import com.instacart.formula.rxjava3.RxAction
+import com.instacart.formula.rxjava3.RxJavaRuntime
 import com.instacart.formula.subjects.ChildActionFiresParentEventOnStart
 import com.instacart.formula.subjects.ChildMessageNoParentStateChange
 import com.instacart.formula.subjects.ChildMessageTriggersEventTransitionInParent
@@ -1111,6 +1112,24 @@ class FormulaRuntimeTest(val runtime: TestableRuntime, val name: String) {
         assertThat(error).isInstanceOf(IllegalStateException::class.java)
     }
 
+    @Test fun `adding duplicate child logs an exception`() {
+        val logs = mutableListOf<String>()
+        RxJavaRuntime.setDefaultErrorHandler { logs.add(it.localizedMessage) }
+
+        val result = Try {
+            val formula = DynamicParentFormula()
+            runtime.test(formula, Unit)
+                .output { addChild(TestKey("1")) }
+                .output { addChild(TestKey("1")) }
+        }
+
+        val error = result.errorOrNull()?.cause
+        assertThat(error).isNull()
+        assertThat(logs).hasSize(1)
+        val log = logs.first()
+        assertThat(log).isEqualTo("Duplicate child key: TestKey(id=1)")
+    }
+
     @Test
     fun `parent removal triggers childs terminate message`() {
         val terminateFormula = TerminateFormula()