Here is a list of cards part of the non-trump suit:
Ace Ten King Nine Eight Seven
Notice that there are only 6 cards. If you have 2 (or more) of them, the chances that every other player having the same suit is rather minimal (or impossible if you are playing 5 hand and you have 3 cards of one suit).
In other words, let's say you have the Ace, 8 and 7 of hearts. If you lead out the hearts suit, you should expect that someone is going to trump it. (There are only 3 hearts left, and 4 players!) Unless you know otherwise, you generally wouldn't lead the ace or ten if you have a long suit. Lead out a smaller card and save that Ace to smear it later. (And chances are, on a long suit, that that Ace WILL become schmear for someone..)