Bug fix: export with -J could fail when two files had the same content.

Exporting is done inside a call to writeLockDbWhile which guarantees there
is only one process uploading to a given ExportLocation.

Command/Export.hs

@@ -283,7 +283,11 @@ performExport r db ek af contentsha loc allfilledvar = do
 	sent <- tryNonAsync $ case ek of
 		AnnexKey k -> ifM (inAnnex k)
 			( notifyTransfer Upload af $
-				upload' (uuid r) k af stdRetry $ \pm -> do
+				-- alwaysUpload because the same key
+				-- could be used for more than one export
+				-- location, and concurrently uploading
+				-- of the content should still be allowed.
+				alwaysUpload (uuid r) k af stdRetry $ \pm -> do
 					let rollback = void $
 						performUnexport r db [ek] loc
 					sendAnnex k rollback $ \f ->