[DOC] Fix double-word typos in comments

Found via `grep` for repeated words.

* set.c: Fix "or or"
* include/ruby/internal/symbol.h: Fix "is is"
* include/ruby/internal/ctype.h: Fix "in in"

Author: aguspe

include/ruby/internal/ctype.h

@@ -498,7 +498,7 @@ RBIMPL_ATTR_ARTIFICIAL()
  * Our own locale-insensitive version of `tolower(3)`.
  *
  * @param[in]  c          Byte in question to convert.
- * @retval     c          The  byte is  not listed  in in  IEEE 1003.1  section
+ * @retval     c          The  byte is  not listed  in  IEEE 1003.1  section
  *                        7.3.1.1 "upper".
  * @retval     otherwise  Byte converted  using the map defined  in IEEE 1003.1
  *                        section 7.3.1 "tolower".

include/ruby/internal/symbol.h

@@ -187,7 +187,7 @@ ID rb_check_id(volatile VALUE *namep);
  *
  * :FIXME:  Can anyone  tell us  what is  the difference  between this  one and
  * rb_intern_str()?  As far as @shyouhei reads the implementation it seems what
- * rb_to_id() does is  is just waste some CPU time,  then call rb_intern_str().
+ * rb_to_id() does is just waste some CPU time,  then call rb_intern_str().
  * He hopes he is wrong.
  */
 ID rb_to_id(VALUE str);

set.c

@@ -1766,7 +1766,7 @@ set_i_disjoint(VALUE set, VALUE other)
  *    set <=> other -> -1, 0, 1, or nil
  *
  *  Returns 0 if the set are equal, -1 / 1 if the set is a
- *  proper subset / superset of the given set, or or nil if
+ *  proper subset / superset of the given set, or nil if
  *  they both have unique elements.
  */
 static VALUE