Do not allow folder contents to have duplicate names.

src/main/kotlin/filesystem/FSCreator.kt

@@ -10,16 +10,22 @@ class FSCreator {
     /**
      * Create entry, leaving existing folders' contents, but overwriting existing files.
      * @throws CyclicFolderException Cyclic folders cannot be created.
+     * @throws DuplicateEntryNameException A folder or sub-folder contains entries with duplicate names.
      */
-    @Throws(CyclicFolderException::class)
+    @Throws(CyclicFolderException::class, DuplicateEntryNameException::class)
     fun create(
         entryToCreate: FSEntry,
         destination: String,
     ) {
         // No point in running anything if we know the input is invalid.
-        if (entryToCreate is FSFolder && entryToCreate.isCyclic()) {
+        if (entryToCreate is FSFolder) {
+            if (entryToCreate.isCyclic()) {
                 throw CyclicFolderException()
             }
+            if (entryToCreate.deepHasDuplicateNames()) {
+                throw DuplicateEntryNameException()
+            }
+        }
 
         val queue =
             ArrayDeque(
@@ -47,3 +53,4 @@ class FSCreator {
 }
 
 class CyclicFolderException : Exception("Cyclic FSFolders are not supported")
+class DuplicateEntryNameException : Exception("Folder contains entries with duplicate names")

src/main/kotlin/filesystem/FSEntry.kt

@@ -34,6 +34,26 @@ class FSFolder(name: String, val entries: List<FSEntry>) : FSEntry(name) {
         }
         return false
     }
+
+    /**
+     * Check whether a folder contains multiple entries with the same name.
+     */
+    fun hasDuplicateNames(): Boolean {
+        val seen = HashSet<String>()
+        return entries.any { !seen.add(it.name) }
+    }
+
+    internal fun deepHasDuplicateNames(): Boolean {
+        val queue = ArrayDeque(listOf(this))
+        while (queue.isNotEmpty()) {
+            val entry = queue.removeFirst()
+            if (entry.hasDuplicateNames()) {
+                return true
+            }
+            queue.addAll(entry.entries.mapNotNull { it as? FSFolder })
+        }
+        return false
+    }
 }
 
 class InvalidEntryNameException(name: String) : Exception("Invalid FSEntry name: '$name'")