--- a/src/testdir/test_options.vim
+++ b/src/testdir/test_options.vim
@@ -1,5 +1,6 @@
 " Test for options
 
+source shared.vim
 source check.vim
 source view_util.vim
 
@@ -587,6 +588,35 @@ func Test_backupskip()
     endif
   endfor
 
+  " Duplicates from environment variables should be filtered out (option has
+  " P_NODUP).  Run this in a separate instance and write v:errors in a file,
+  " so that we see what happens on startup.
+  let after =<< trim [CODE]
+      let bsklist = split(&backupskip, ',')
+      call assert_equal(uniq(copy(bsklist)), bsklist)
+      call writefile(['errors:'] + v:errors, 'Xtestout')
+      qall
+  [CODE]
+  call writefile(after, 'Xafter')
+  let cmd = GetVimProg() . ' --not-a-term -S Xafter --cmd "set enc=utf8"'
+
+  let saveenv = {}
+  for var in ['TMPDIR', 'TMP', 'TEMP']
+    let saveenv[var] = getenv(var)
+    call setenv(var, '/duplicate/path')
+  endfor
+
+  exe 'silent !' . cmd
+  call assert_equal(['errors:'], readfile('Xtestout'))
+
+  " restore environment variables
+  for var in ['TMPDIR', 'TMP', 'TEMP']
+    call setenv(var, saveenv[var])
+  endfor
+
+  call delete('Xtestout')
+  call delete('Xafter')
+
   " Duplicates should be filtered out (option has P_NODUP)
   let backupskip = &backupskip
   set backupskip=