export: Fix false positive in export conflict detection

It occurred when the same tree was exported by multiple clones. nub out
identical trees.

This commit was sponsored by Jochen Bartl on Patreon.

CHANGELOG

@@ -10,6 +10,8 @@ git-annex (6.20180927) UNRELEASED; urgency=medium
   * SETURLPRESENT, SETURIPRESENT, SETURLMISSING, and SETURIMISSING
     used to update the presence information of the external special remote
     that called them; this was not documented behavior and is no longer done.
+  * export: Fix false positive in export conflict detection, that occurred
+    when the same tree was exported by multiple clones.
 
  -- Joey Hess <id@joeyh.name>  Thu, 27 Sep 2018 15:27:20 -0400
 

Command/Export.hs

@@ -113,7 +113,7 @@ changeExport r ea db new = do
 	-- When there was an export conflict, this resolves it.
 	--
 	-- The ExportTree is also updated here to reflect the new tree.
-	case map exportedTreeish old of
+	case nub (map exportedTreeish old) of
 		[] -> updateExportTree db emptyTree new
 		[oldtreesha] -> do
 			diffmap <- mkDiffMap oldtreesha new db