Differences in Salary?

I recently found out that somebody at my company that has the exact same position as me is making more money than I am. It’s not much, but my salary being adjusted to theirs would come out to about a 2.8% raise. I work for a relatively small company, so I didn’t think this kind of discrepancy would occur. My co-worker and I were hired at the same time, started the same day at the same entry level position, and were even recently promoted on the same day to the next level in our group. What should I do about this? Do I bring it up to my boss, or HR? Should I bring it up at all, or will it make me sound petty? I would appreciate any advice with this topic seeing as it is often seen as a taboo subject to discuss in the workplace.