It is a truism that first impressions count. What is interesting is that this applies to smelling as much as anything else.
In his book, What the Nose Knows, Avery Gilbert talks about the art of the sniff and the science behind it:
“David Laing systematically controlled sniffing to see how it affected a person’s ability to detect and describe a smell. Sometimes he allowed his subjects to sniff with their natural pattern; other times he told them exactly how many sniffs to take, how long to wait between sniffs, or how big a sniff to take. When subjects were limited to a single sniff, they took one that resembled the first in a natural sniffing episode. Whether the sniff was the first-and-only or the first-of-many, it did not appear to vary with odor strength. After many experiments he could state his findings in a nutshell: “a single natural sniff provides as much information about the presence and intensity of an odour as do seven or more sniffs.” A natural first sniff can’t be beat. (For the technically minded, the optimum sniff has an inhalation rate of 30 liters per minute, a volume of 200 cubic centimeters, and a minimum duration of .40 to .45 seconds.)”
So you don’t need to sniff twice to find out what it is you’re smelling and how intense it is. You may want to take more sniffs though, to get a more nuanced impression, especially if there is a decent time interval between sniffs.