First, as noted above, you can not have two columns with the same name in the same table. Since you must add the column and alter it, then simply enhance the script above:
IF NOT EXISTS
( SELECT column_name FROM INFORMATION_SCHEMA.COLUMNS
WHERE table_name = 'MyTable' AND column_name = 'MyNewColumn'
)
BEGIN
ALTER TABLE MyTable ADD MyNewColumn INT NOT NULL default 1;
END
ELSE
BEGIN
END