--- a/net/neoforged/art/internal/RenamerImpl.java
+++ b/net/neoforged/art/internal/RenamerImpl.java
@@ -35,7 +_,7 @@
 import net.neoforged.cliutils.progress.ProgressReporter;
 import org.objectweb.asm.Opcodes;
 
-class RenamerImpl implements Renamer {
+public class RenamerImpl implements Renamer { // Paper - public
     private static final ProgressReporter PROGRESS = ProgressReporter.getDefault();
     static final int MAX_ASM_VERSION = Opcodes.ASM9;
     private static final String MANIFEST_NAME = "META-INF/MANIFEST.MF";
@@ -75,6 +_,11 @@
 
     @Override
     public void run(File input, File output) {
+        // Paper start - Add remappingSelf
+        this.run(input, output, false);
+    }
+    public void run(File input, File output, boolean remappingSelf) {
+        // Paper end
         if (!this.setup)
             this.setup();
 
@@ -105,10 +_,10 @@
                 String name = e.getName();
                 byte[] data;
                 try (InputStream entryInput = in.getInputStream(e)) {
-                    data = readAllBytes(entryInput, e.getSize());
+                    data = entryInput.readAllBytes(); // Paper - Use readAllBytes
                 }
 
-                if (name.endsWith(".class"))
+                if (name.endsWith(".class") && !name.contains("META-INF/")) // Paper - Skip META-INF entries
                     oldEntries.add(ClassEntry.create(name, e.getTime(), data));
                 else if (name.equals(MANIFEST_NAME))
                     oldEntries.add(ManifestEntry.create(e.getTime(), data));
@@ -163,15 +_,29 @@
             List<Entry> newEntries = async.invokeAll(oldEntries, Entry::getName, this::processEntry);
 
             logger.accept("Adding extras");
-            transformers.forEach(t -> newEntries.addAll(t.getExtras()));
+            // Paper start - I'm pretty sure the duplicates are because the input is already on the classpath
+            List<Entry> finalNewEntries = newEntries;
+            transformers.forEach(t -> finalNewEntries.addAll(t.getExtras()));
 
             Set<String> seen = new HashSet<>();
+            if (remappingSelf) {
+                // deduplicate
+                List<Entry> n = new ArrayList<>();
+                for (final Entry e : newEntries) {
+                    if (seen.add(e.getName())) {
+                        n.add(e);
+                    }
+                }
+                newEntries = n;
+            } else {
             String dupes = newEntries.stream().map(Entry::getName)
                 .filter(n -> !seen.add(n))
                 .sorted()
                 .collect(Collectors.joining(", "));
             if (!dupes.isEmpty())
                 throw new IllegalStateException("Duplicate entries detected: " + dupes);
+            }
+            // Paper end
 
             // We care about stable output, so sort, and single thread write.
             logger.accept("Sorting");