A true double standard is one of the clearest distortions of truth in human behavior. It occurs when two people (or two situations) are judged by different rules even though the circumstances are essentially the same. In simple terms: A double standard is when the rule changes depending on who is being judged. It is not merely disagreement. It is inconsistent judgment.
A person might say “lying is wrong”, but excuse their own lie while condemning someone else’s. The principle remains the same, but the application changes based on identity, emotion, or advantage. This is why double standards are so corrosive: they break the relationship between principle and behavior.
A real double standard has three parts: stated principle, two similar situations, and different judgments applied. The rule becomes flexible only when it benefits the person applying it. This usually happens when personal truth replaces objective evaluation. People begin protecting their identity instead of pursuing reality. The moment the mind shifts from what is true to how do I protect myself, double standards begin.
Double standards rarely start from malice. They usually come from psychological protection. Several forces push people into them: ego protection, tribal thinking, emotional bias, and shame avoidance.
Double standards are used in relationships, arguments, politics, and social conflicts. Often without realizing it. They become moral weaponization as a person holds another to a rule they refuse to follow. They can also be selective accountability where one person must apologize and the other never does. They can be used as narrative control where facts are interpreted differently depending on who is involved. Finally, they can be used to establish relationship power where one partner becomes the permanent judge. Double standards create damage by eroding trust, creating disrespect, and avoiding self-honesty.
To stop using double standards a person must do some serious internal work. It requires radical self-honesty by asking if I would judge another the same way I would judge myself. It also requires putting principle before identity. Instead of assessing who is right and figuring out what rule should apply to everyone. Fairness can only exist when principles apply universally. Otherwise they are not principles. They are preferences.
A person must exercise humility. Humility is, in fact, the antidote to double standards, as humility accepts this uncomfortable truth: I am capable of the same flaws I criticize in others. When people accept that reality, their judgment becomes more balanced. This is the same humility required for real behavioral change. 
Ask yourself: If I did this same thing how would I justify it? How would I judge it? If the answer differs from your response, then a double standard is present. BUT this requires evaluating from a place of truth and the confrontation of perspective from the point of reality.