GCMemcardManager: Detect attempt to import multiple save files with the same internal name.

Source/Core/Core/HW/GCMemcard/GCMemcardUtils.cpp

@@ -72,6 +72,19 @@ bool HasSameIdentity(const DEntry& lhs, const DEntry& rhs)
   return true;
 }
 
+bool HasDuplicateIdentity(const std::vector<Savefile>& savefiles)
+{
+  for (size_t i = 0; i < savefiles.size(); ++i)
+  {
+    for (size_t j = i + 1; j < savefiles.size(); ++j)
+    {
+      if (HasSameIdentity(savefiles[i].dir_entry, savefiles[j].dir_entry))
+        return true;
+    }
+  }
+  return false;
+}
+
 static void ByteswapDEntrySavHeader(std::array<u8, DENTRY_SIZE>& entry)
 {
   // several bytes in SAV are swapped compared to the internal memory card format

Source/Core/Core/HW/GCMemcard/GCMemcardUtils.h

@@ -13,6 +13,9 @@ namespace Memcard
 {
 bool HasSameIdentity(const DEntry& lhs, const DEntry& rhs);
 
+// Check if any two given savefiles have the same identity.
+bool HasDuplicateIdentity(const std::vector<Savefile>& savefiles);
+
 enum class ReadSavefileErrorCode
 {
   OpenFileFail,

Source/Core/DolphinQt/GCMemcardManager.cpp

@@ -515,6 +515,12 @@ void GCMemcardManager::ImportFiles(int slot, const std::vector<Memcard::Savefile
            "", static_cast<int>(number_of_blocks)));
   }
 
+  if (Memcard::HasDuplicateIdentity(savefiles))
+  {
+    error_messages.push_back(
+        tr("At least two of the selected save files have the same internal filename."));
+  }
+
   for (const Memcard::Savefile& savefile : savefiles)
   {
     if (card->TitlePresent(savefile.dir_entry))