Paid Less And Pissed Off
Question: I just found out that I'm paid 20% less than the dude sitting next to me who does the same job. Who do I punch in the face?
Answer: You don't mention whether you're female, but that's fine. My advice would be the same whether you're a man or a woman.
Put your annoyance in your pocket for a moment and ask yourself these important contextualizing questions:
Has "dude" been there longer than you? A few end-of-year merit increases could have led to a 20% gap by the time you joined. Also, did you do due diligence on market rates before accepting your initial offer? Most people don't ask whether their official offer has upward wiggle room, which is a mistake because most often it does. Or, if you've been there a while, have you provided your boss with winning examples of why you deserve a raise?
If dude is making more than you, it's probably because he asked for more than you. Meaning: You should be upset with yourself, not your colleague, boss or HR department. If you have pushed for more, haven't received it, and demonstrate the same level of skill as your colleague, then yes, you have every right to be pissed.
Either way, a productive way of getting closer to where you should be is to set a time on your boss' calendar to discuss your role. Before you meet, take a comprehensive look at what the market is paying people in similar job functions (if it's below what your colleague is making and you know for a fact that he makes 20% more, "skew up" to that). Discuss the range in full when you meet your boss, backing it up with ways that you're not only killing it now, but could bring even more to the table if you took on projects A, B and C that a raise would support.
Whatever you do, don't list your colleague's level of pay as a reason for you deserving the same. While that may feel fair to you, to your boss it will come across like a kid complaining to mom that big brother got two extra cookies.