I presume you bought them used or custom ordered them.

My circumstance was, I bought some used AP2 irons and the seller mentioned that they would have serial numbers on every club because they had been custom ordered and that Titleist would list the serial number on every club, unlike a standard production set with the number just on the 6 iron.

I called Titleist to make sure the clubs fit the specs and they did, so I was OK with it.

With my custom ordered 712 AP2 4-PW, the same serial number is listed on each iron.  Additionally, when I ordered the 3i and W wedge separately, both had different serial number on them in addition to the Security Label.  

Therefore, I wouldn't be overly concerned about the authenticity.  However, if you are concerned, I would recommend locating the Security Label, which is found exclusively on the 6i, backside, just below the grip.