patch 8.2.3888: the argument list may contain duplicates

Problem:    The argument list may contain duplicates.
Solution:   Add the :argdedeupe command. (Nir Lichtman, closes #6235)
diff --git a/src/ex_cmds.h b/src/ex_cmds.h
index 044580a..3cdb656 100644
--- a/src/ex_cmds.h
+++ b/src/ex_cmds.h
@@ -148,6 +148,9 @@
 EXCMD(CMD_argdo,	"argdo",	ex_listdo,
 	EX_BANG|EX_NEEDARG|EX_EXTRA|EX_NOTRLCOM|EX_RANGE|EX_DFLALL|EX_EXPAND,
 	ADDR_ARGUMENTS),
+EXCMD(CMD_argdedupe,	"argdedupe",	ex_argdedupe,
+	EX_TRLBAR,
+	ADDR_NONE),
 EXCMD(CMD_argedit,	"argedit",	ex_argedit,
 	EX_BANG|EX_NEEDARG|EX_RANGE|EX_ZEROR|EX_FILES|EX_CMDARG|EX_ARGOPT|EX_TRLBAR,
 	ADDR_ARGUMENTS),