Not all sin is equally grievous. In some cases, it may even be more helpful, pastorally, to speak of "error" or "poor understanding" rather than of "sin," which usually sounds like an accusation of willful defiance.
A person who knows it is wrong to be baptized again but does so anyway in order to please men, or out of superstition, or for the novelty of the experience ("Here I am at the Jordan River, and I want the Jordan-baptism thrill/blessing") is sinning quite grievously. He is willfully searing his conscience and defying God.
A person who has not been taught that baptism is a one-time event, and who gets baptized again either for the thrill or because of doubts or out of superstition or due to carelessness in not learning about the rite, is not sinning as egregiously. But he is still sinning in his thrill-seeking or doubt or superstition or carelessness.
A person who is a convinced credobaptist and believes that obedience to God requires a valid (re)baptism is still in error, and he too has surely come to that error because of some hard-to-see-and-eradicate sin in his life. If we speak honestly, we must say there is probably some lurking doubt or legalism or arrogance or some other hard-to-see sin that has contributed to his deception. BUT... his complicity is less than that of the person who is willfully or carelessly sinning. Don't we all have lurking doubt and legalism and arrogance that lead us to error and sinful behavior? Pastorally, the best way to counsel such a person is surely not to treat him as a willful sinner who just needs to buckle down and submit. Rather, it is to affirm with joy the rightness of our brother's desire to obey God, and then to patiently teach him the glories of grace and covenant and God's time-defying decrees, and to compassionately examine with him the idols of his heart, until his error dissolves. That process will include a repentance that gets at those hard-to-see sins, but it takes place on a deeper level than where the open defiance or carelessness exist, and so it is often unhelpful to speak of them as if they were the same thing.
So on the one hand, I know there is always sin where there is error. But more importantly, I know that I too am still in the process of discovering and repenting of deep-seated sin that leads to much error in my life. And so, I hesitate to speak of my credobaptist brothers and sisters as if they were people who are practicing sin in ways that I am not. And I remind myself that Scripture prefers to speak of all of us as those who are obeying our Father, even though we are still learning and we do it imperfectly.