Don't add explanation twice (#18779)

Fixes #18774

The improvement was #14357

Author: nicolasstucki

sbt-bridge/src/dotty/tools/xsbt/DelegatingReporter.java

@@ -48,22 +48,17 @@ public void printSummary(Context ctx) {
   public void doReport(Diagnostic dia, Context ctx) {
     Severity severity = severityOf(dia.level());
     Position position = positionOf(dia.pos().nonInlined());
-
-    StringBuilder rendered = new StringBuilder();
-    rendered.append(messageAndPos(dia, ctx));
     Message message = dia.msg();
-    StringBuilder messageBuilder = new StringBuilder();
-    messageBuilder.append(message.message());
+    String text;
+    if (Diagnostic.shouldExplain(dia, ctx) && !message.explanation().isEmpty())
+      text = message.message() + System.lineSeparator() + explanation(message, ctx);
+    else
+      text = message.message();
+    String rendered = messageAndPos(dia, ctx);
     String diagnosticCode = String.valueOf(message.errorId().errorNumber());
-    boolean shouldExplain = Diagnostic.shouldExplain(dia, ctx);
     List<CodeAction> actions = CollectionConverters.asJava(message.actions(ctx));
-    if (shouldExplain && !message.explanation().isEmpty()) {
-      rendered.append(explanation(message, ctx));
-      messageBuilder.append(System.lineSeparator()).append(explanation(message, ctx));
-    }
-
-    delegate.log(new Problem(position, messageBuilder.toString(), severity, rendered.toString(), diagnosticCode, actions,
-      lookupVirtualFileId));
+    Problem problem = new Problem(position, text, severity, rendered, diagnosticCode, actions, lookupVirtualFileId);
+    delegate.log(problem);
   }
 
   public void reportBasicWarning(String message) {