Fixed typos

- Removed duplicate words, like "the the" and "to to"
- Removed duplicate lines (one double sort line found in mysql.cc)
- Fixed some typos found while searching for duplicate words.

Command used to find duplicate words:
egrep -rI "\s([a-zA-Z]+)\s+\1\s" | grep -v param

Thanks to Artjoms Rimdjonoks for the command and pointing out the
spelling errors.

sql/sql_type.h

@@ -4192,7 +4192,7 @@ public:
     @retval
       NULL      on error
     @retval
-      non-NULL  a pointer to a a valid string on success
+      non-NULL  a pointer to a valid string on success
   */
   virtual String *print_item_value(THD *thd, Item *item, String *str) const= 0;