patch 8.2.1666: the initial value of 'backupskip' can have duplicate items

Problem:    The initial value of 'backupskip' can have duplicate items.
Solution:   Remove duplicates, like when it is set later. (Tom Ryder,
            closes #6940)
diff --git a/src/version.c b/src/version.c
index e0afdf8..596f38d 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1666,
+/**/
     1665,
 /**/
     1664,