The following example is from the book Oxford Guide to English Grammar describing Not usage
Beggars are a not unusual sight on the streets of London.
Should that be
Beggars are not an unusual sight on the streets of London.
"a not unusual" in the first sentence seems not idiomatic or right to me.
Can anybody explain this to me? How a native speaker would say this?
Also I want to know the two negative words "not unusual" is all right to use?