vim-patch:8.2.3888: the argument list may contain duplicates (#19795)

Problem:    The argument list may contain duplicates.
Solution:   Add the :argdedeupe command. (Nir Lichtman, closes vim/vim#6235)
73a024209c

Use latest index.txt :argdedupe doc from Vim.

src/nvim/ex_cmds.lua

@@ -106,6 +106,12 @@ module.cmds = {
     addr_type='ADDR_ARGUMENTS',
     func='ex_listdo',
   },
+  {
+    command='argdedupe',
+    flags=TRLBAR,
+    addr_type='ADDR_NONE',
+    func='ex_argdedupe',
+  },
   {
     command='argedit',
     flags=bit.bor(BANG, NEEDARG, RANGE, ZEROR, FILES, CMDARG, ARGOPT, TRLBAR),