patch 8.1.2159: some mappings are listed twice

Problem:    Some mappings are listed twice.
Solution:   Skip mappings duplicated for modifyOtherKeys. (closes #5064)
diff --git a/src/testdir/test_mapping.vim b/src/testdir/test_mapping.vim
index 93a4d4b..f62d575 100644
--- a/src/testdir/test_mapping.vim
+++ b/src/testdir/test_mapping.vim
@@ -442,3 +442,19 @@
   call delete('Xtest.vim')
   exe buf .. 'bwipe!'
 endfunc
+
+func Test_list_mappings()
+  inoremap <C-M> CtrlM
+  inoremap <A-S> AltS
+  inoremap <S-/> ShiftSlash
+  call assert_equal([
+	\ 'i  <S-/>       * ShiftSlash',
+	\ 'i  <M-S>       * AltS',
+	\ 'i  <C-M>       * CtrlM',
+	\], execute('imap')->trim()->split("\n"))
+  iunmap <C-M>
+  iunmap <A-S>
+  call assert_equal(['i  <S-/>       * ShiftSlash'], execute('imap')->trim()->split("\n"))
+  iunmap <S-/>
+  call assert_equal(['No mapping found'], execute('imap')->trim()->split("\n"))
+endfunc