Symlinking final file failed is a common issue :/.

apt-pkg/acquire-item.cc

@@ -3071,6 +3071,7 @@ void pkgAcqIndex::StageDownloadDone(string const &Message)
       // copy FinalFile into partial/ so that we check the hash again
       string const FinalFile = GetExistingFilename(GetFinalFileNameFromURI(Target.URI));
       DestFile = GetKeepCompressedFileName(GetPartialFileNameFromURI(Target.URI), Target);
+      unlink(DestFile.c_str());
       if (symlink(FinalFile.c_str(), DestFile.c_str()) != 0)
 	 _error->WarningE("pkgAcqIndex::StageDownloadDone", "Symlinking final file %s back to %s failed", FinalFile.c_str(), DestFile.c_str());
       else