It's not bad to map letters in the clear text to the same letter in the cypher text - as long as it's not more often than the statistical average.
No, it isn't bad - it is just wrong. To quote wikipedia on cryptogram, "...each letter is replaced by a different letter." Perhaps you don't know anyone who likes to do these puzzles but they would be extremely upset if they found one of the letters mapped to itself.
Cheers - L~R