Fix issue with multiple async status files causing a hard error.

Multiple status files were being created by asynchronous archiving if a high-level error occurred after one or more WAL segments had already been transferred successfully.  Error files were being written for every file in the queue regardless of whether it had already succeeded.  To fix this, add an option to skip writing error files when an ok file already exists.

There are other situations where both files might exist (various fsync and filesystem error scenarios) so it seems best to retry in the case that multiple status files are found rather than throwing a hard error (which then means that archiving is completely stuck).  In the case of multiple status files, a warning will be logged to alert the user that something unusual is happening and the command will be retried.

Reported by fpa-postgres, Joe Ayers, Douglas J Hunley.

test/lib/pgBackRestTest/Module/Archive/ArchiveCommonPerlTest.pm

@@ -199,6 +199,15 @@ sub run
         # Generate a valid warning ok
         archiveAsyncStatusWrite(WAL_STATUS_OK, $strSpoolPath, $strSegment, 0, 'Test Warning');
 
+        # Skip error when an ok file already exists
+        #---------------------------------------------------------------------------------------------------------------------------
+        archiveAsyncStatusWrite(
+            WAL_STATUS_ERROR, $strSpoolPath, $strSegment, ERROR_ARCHIVE_DUPLICATE,
+            "WAL segment ${strSegment} already exists in the archive", true);
+
+        $self->testResult(
+            $self->storageTest()->exists("${strSpoolPath}/${strSegment}.error"), false, "error file should not exist");
+
         #---------------------------------------------------------------------------------------------------------------------------
         # Generate an invalid error
         $self->testException(