It's about someone who can't be trusted, someone who hurt the speaker in the past and is now coming back and insisting she has changed. The speaker knows better, realizing that she is lying to herself as well as him, putting on another personality to avoid dealing with the things she's done.
The chorus suggests that the speaker may be "taking his chances" on a friendship/relationship with this person again, but he thinks that since now he knows she is inconsistent and two-faced, he'll be able to question her ("second guess it") and will avoid getting hurt.
It's about someone who can't be trusted, someone who hurt the speaker in the past and is now coming back and insisting she has changed. The speaker knows better, realizing that she is lying to herself as well as him, putting on another personality to avoid dealing with the things she's done.
The chorus suggests that the speaker may be "taking his chances" on a friendship/relationship with this person again, but he thinks that since now he knows she is inconsistent and two-faced, he'll be able to question her ("second guess it") and will avoid getting hurt.