Their integrity will guide them. Unfaithfulness and duplicity both imply multiple standards or goals. Their action depends on what they believe will benefit them most in a given situation. You may be working with someone like this and they will cooperate with you up to a point. When they see another desirable goal, or their personal interest becomes more important than what you are working on, they abandon you and do something else, even if it means undermining the work they were doing with you.
Duplicity also includes deception.
The person in our example may have been making you think they wanted to help you, but all along they had something else in mind. They very likely were using you to get what they wanted. One remains focused, adhering to a high standard and consistently applying themselves to a specific end. All of their efforts ultimately are directed toward their goal. The other person has more than one standard or purpose, and they jump from one to the other, depending on circumstances.