Fix duplicate plugins in the plugin tree after update

scm-webapp/src/main/java/sonia/scm/plugin/ExplodedSmp.java

@@ -31,6 +31,7 @@ import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
 import java.nio.file.Path;
+import java.util.Objects;
 
 //~--- JDK imports ------------------------------------------------------------
 
@@ -105,6 +106,25 @@ public final class ExplodedSmp
     return plugin;
   }
 
+  @Override
+  public boolean equals(Object o) {
+    if (this == o) return true;
+    if (o == null || getClass() != o.getClass()) return false;
+    ExplodedSmp that = (ExplodedSmp) o;
+    return path.equals(that.path);
+  }
+
+  @Override
+  public int hashCode() {
+    return Objects.hash(path);
+  }
+
+  @Override
+  public String toString() {
+    PluginInformation information = plugin.getInformation();
+    return information.getName() + "@" + information.getVersion() + " (" + path + ")";
+  }
+
   //~--- inner classes --------------------------------------------------------
 
   /**

scm-webapp/src/main/java/sonia/scm/plugin/PluginProcessor.java

@@ -190,8 +190,14 @@ public final class PluginProcessor
     return ImmutableSet.copyOf(wrappers);
   }
 
-  private ImmutableSet<ExplodedSmp> concat(Set<ExplodedSmp> installedPlugins, Set<ExplodedSmp> newlyInstalledPlugins) {
-    return ImmutableSet.<ExplodedSmp>builder().addAll(installedPlugins).addAll(newlyInstalledPlugins).build();
+  private Set<ExplodedSmp> concat(Set<ExplodedSmp> installedPlugins, Set<ExplodedSmp> newlyInstalledPlugins) {
+    // We first add all newly installed plugins,
+    // after that we add the missing plugins, which are already installed.
+    // ExplodedSmp is equal by its path, so duplicates (updates) are not in the result.
+    return ImmutableSet.<ExplodedSmp>builder()
+      .addAll(newlyInstalledPlugins)
+      .addAll(installedPlugins)
+      .build();
   }
 
   private Set<ExplodedSmp> installPending(Set<ExplodedSmp> installedPlugins) throws IOException {