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_statistics.h

@@ -359,7 +359,7 @@ public:
 
 
 /*
-  This is used to collect the the basic statistics from a Unique object:
+  This is used to collect the basic statistics from a Unique object:
    - count of values
    - count of distinct values
    - count of distinct values that have occurred only once