Correct corrupted downloaded zip

Prevent the zipping of twice the same file (resulting in an error and a corrupted zip on Linux) when the cover is embedded in tags

airsonic-main/src/main/java/org/airsonic/player/controller/DownloadController.java

@@ -207,6 +207,8 @@ public class DownloadController implements LastModified {
      * @param zipFileName  The name of the resulting zip file.   @throws IOException If an I/O error occurs.
      */
     private void downloadFiles(HttpServletResponse response, TransferStatus status, List<MediaFile> files, int[] indexes, File coverArtFile, HttpRange range, String zipFileName) throws IOException {
+        boolean cover_embedded = false;
+
         if (indexes != null && indexes.length == 1) {
             downloadFile(response, status, files.get(indexes[0]).getFile(), range);
             return;
@@ -232,8 +234,13 @@ public class DownloadController implements LastModified {
 
         for (MediaFile mediaFile : filesToDownload) {
             zip(out, mediaFile.getParentFile(), mediaFile.getFile(), status, range);
+            if (coverArtFile != null && coverArtFile.exists()) {
+                if (mediaFile.getFile().getCanonicalPath() == coverArtFile.getCanonicalPath()) {
+                    cover_embedded = true;
+                }
+            }
         }
-        if (coverArtFile != null && coverArtFile.exists()) {
+        if (coverArtFile != null && coverArtFile.exists() && cover_embedded == false) {
             zip(out, coverArtFile.getParentFile(), coverArtFile, status, range);
         }