all: fix typos where we repeat repeat ourselves

Found with the regex `\b([A-Za-z]+) \1\b`.

Updates #cleanup

Change-Id: I4cc51784d9b6437d3d0c66b531828707f87f7fd5
Signed-off-by: Alex Chan <alexc@tailscale.com>

types/opt/bool.go

@@ -13,7 +13,7 @@ import (
 // is either "true", "false", or the empty string to mean unset.
 //
 // As a special case, the underlying string may also be the string
-// "unset" as as a synonym for the empty string. This lets the
+// "unset" as a synonym for the empty string. This lets the
 // explicit unset value be exchanged over an encoding/json "omitempty"
 // field without it being dropped.
 type Bool string