Prohibit duplicate enum member IDs (names)

Silently allowing duplicate enum member IDs causes problems in real
formats, see
- kaitai-io/kaitai_struct_formats#345
- kaitai-io/kaitai_struct_formats#346
- kaitai-io/kaitai_struct_formats#421 (comment)
- kaitai-io/kaitai_struct_formats#448

shared/src/main/scala/io/kaitai/struct/format/EnumSpec.scala

@@ -1,5 +1,9 @@
 package io.kaitai.struct.format
 
+import io.kaitai.struct.problems.KSYParseError
+
+import scala.collection.mutable
+
 case class EnumSpec(map: Map[Long, EnumValueSpec]) {
   var name = List[String]()
 
@@ -19,10 +23,18 @@ case class EnumSpec(map: Map[Long, EnumValueSpec]) {
 object EnumSpec {
   def fromYaml(src: Any, path: List[String]): EnumSpec = {
     val srcMap = ParseUtils.asMap(src, path)
+    val memberNameMap = mutable.Map[String, Long]()
     EnumSpec(srcMap.map { case (id, desc) =>
       val idLong = ParseUtils.asLong(id, path)
       val value = EnumValueSpec.fromYaml(desc, path ++ List(idLong.toString))
 
+      memberNameMap.get(value.name).foreach { (prevIdLong) =>
+        throw KSYParseError.withText(
+          s"duplicate enum member ID: '${value.name}', previously defined at /${(path ++ List(prevIdLong.toString)).mkString("/")}",
+          path ++ List(idLong.toString)
+        )
+      }
+      memberNameMap.put(value.name, idLong)
       idLong -> value
     })
   }