OK to preview twice, just keep it short. From the PhotoReading book:
When done effectively, previewing is short and
sweet—about 60 to 90 seconds for a book, 60 seconds
for a long report, and as little as 30 seconds for a short
article. During that time, you look over the material to
get a general idea of what will be covered in the book
or document, clarify and refine your purpose, and
decide whether to continue reading or call it quits.
