Split userdev data entry point into it's own provider to fix duplicate mod issue.

src/userdev/java/net/minecraftforge/userdev/ClasspathLocator.java

@@ -67,6 +67,7 @@ public class ClasspathLocator extends AbstractJarFileLocator {
                 if (Files.isDirectory(path))
                     continue;
 
+                LOGGER.debug(CORE, "Found classpath mod: {}", path);
                 this.modCoords.add(path);
             }
         } catch (IOException e) {

src/userdev/java/net/minecraftforge/userdev/FMLUserdevDataLaunchProvider.java

@@ -0,0 +1,61 @@
+/*
+ * Minecraft Forge
+ * Copyright (c) 2016-2019.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation version 2.1
+ * of the License.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+
+package net.minecraftforge.userdev;
+
+import cpw.mods.modlauncher.api.ILaunchHandlerService;
+import cpw.mods.modlauncher.api.ITransformingClassLoader;
+import net.minecraftforge.api.distmarker.Dist;
+
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
+
+import java.util.concurrent.Callable;
+
+import static net.minecraftforge.fml.loading.LogMarkers.CORE;
+
+public class FMLUserdevDataLaunchProvider extends FMLUserdevLaunchProvider implements ILaunchHandlerService
+{
+    private static final Logger LOGGER = LogManager.getLogger();
+
+    @Override
+    public String name()
+    {
+        return "fmluserdevdata";
+    }
+
+    @Override
+    public Callable<Void> launchService(String[] arguments, ITransformingClassLoader launchClassLoader)
+    {
+        return () -> {
+            LOGGER.debug(CORE, "Launching minecraft in {} with arguments {}", launchClassLoader, arguments);
+            super.beforeStart(launchClassLoader);
+            launchClassLoader.addTargetPackageFilter(getPackagePredicate());
+            Thread.currentThread().setContextClassLoader(launchClassLoader.getInstance());
+            Class.forName("net.minecraft.data.Main", true, launchClassLoader.getInstance()).getMethod("main", String[].class).invoke(null, (Object)arguments);
+            return null;
+        };
+    }
+
+    @Override
+    public Dist getDist()
+    {
+        return Dist.CLIENT;
+    }
+}

src/userdev/resources/META-INF/services/cpw.mods.modlauncher.api.ILaunchHandlerService

@@ -1,5 +1,6 @@
 net.minecraftforge.userdev.FMLUserdevClientLaunchProvider
 net.minecraftforge.userdev.FMLUserdevServerLaunchProvider
+net.minecraftforge.userdev.FMLUserdevDataLaunchProvider
 net.minecraftforge.userdev.FMLDevClientLaunchProvider
 net.minecraftforge.userdev.FMLDevServerLaunchProvider
 net.minecraftforge.userdev.FMLDevDataLaunchProvider