diff --git a/zio-json/shared/src/main/scala-2.x/zio/json/macros.scala b/zio-json/shared/src/main/scala-2.x/zio/json/macros.scala
index 3b9e62edf..369bb1e89 100644
--- a/zio-json/shared/src/main/scala-2.x/zio/json/macros.scala
+++ b/zio-json/shared/src/main/scala-2.x/zio/json/macros.scala
@@ -398,6 +398,13 @@ object DeriveJsonDecoder {
     val names = ctx.subtypes.map { p =>
       p.annotations.collectFirst { case jsonHint(name) => name }.getOrElse(jsonHintFormat(p.typeName.short))
     }.toArray
+    if (names.distinct.length != names.length) {
+      val collisions = names.groupBy(identity).collect { case (n, ns) if ns.lengthCompare(1) > 0 => n }
+      throw new AssertionError(
+        s"Case names in ADT ${ctx.typeName.full} must be distinct, " +
+          s"name(s) ${collisions.mkString(",")} are duplicated"
+      )
+    }
     val matrix        = new StringMatrix(names)
     lazy val tcs      = ctx.subtypes.map(_.typeclass).toArray.asInstanceOf[Array[JsonDecoder[Any]]]
     lazy val namesMap = names.zipWithIndex.toMap
diff --git a/zio-json/shared/src/main/scala-3/zio/json/macros.scala b/zio-json/shared/src/main/scala-3/zio/json/macros.scala
index 276e5c36a..99f857f77 100644
--- a/zio-json/shared/src/main/scala-3/zio/json/macros.scala
+++ b/zio-json/shared/src/main/scala-3/zio/json/macros.scala
@@ -404,6 +404,11 @@ sealed class JsonDecoderDerivation(config: JsonCodecConfiguration) extends Deriv
     val names: Array[String] = IArray.genericWrapArray(ctx.subtypes.map { p =>
       p.annotations.collectFirst { case jsonHint(name) => name }.getOrElse(jsonHintFormat(p.typeInfo.short))
     }).toArray
+    if (names.distinct.length != names.length) {
+      val collisions = names.groupBy(identity).collect { case (n, ns) if ns.lengthCompare(1) > 0 => n }
+      throw new AssertionError(s"Case names in ADT ${ctx.typeInfo.full} must be distinct, " +
+        s"name(s) ${collisions.mkString(",")} are duplicated")
+    }
     val matrix: StringMatrix = new StringMatrix(names)
     lazy val tcs: Array[JsonDecoder[Any]] =
       IArray.genericWrapArray(ctx.subtypes.map(_.typeclass)).toArray.asInstanceOf[Array[JsonDecoder[Any]]]
diff --git a/zio-json/shared/src/test/scala/zio/json/DecoderSpec.scala b/zio-json/shared/src/test/scala/zio/json/DecoderSpec.scala
index 37be1314d..7afaeaa80 100644
--- a/zio-json/shared/src/test/scala/zio/json/DecoderSpec.scala
+++ b/zio-json/shared/src/test/scala/zio/json/DecoderSpec.scala
@@ -282,6 +282,20 @@ object DecoderSpec extends ZIOSpecDefault {
           assert("""{"hint":"Child2"}""".fromJson[Parent])(isLeft(equalTo("(invalid disambiguator)"))) &&
           assert("""{"child1":{}}""".fromJson[Parent])(isLeft(equalTo("(missing hint 'hint')")))
         },
+        test("sum with duplicated case names") {
+          for {
+            error <- ZIO.attempt {
+                       sealed trait Fruit
+                       case class Banana(curvature: Double)                extends Fruit
+                       @jsonHint("Banana") case class Apple(color: String) extends Fruit
+                       DeriveJsonDecoder.gen[Fruit]
+                     }.flip
+          } yield assertTrue(
+            error.getMessage.matches(
+              """Case names in ADT zio.json.DecoderSpec.spec(.\$anonfun)?.Fruit must be distinct, name\(s\) Banana are duplicated"""
+            )
+          )
+        },
         test("unicode") {
           assert(""""€🐵🥰"""".fromJson[String])(isRight(equalTo("€🐵🥰")))
         },