My manager was in this position when I was hired. It sucked for them. They asked for a raise based only on what I was paid and "fairness" and were told that I'd used up all the salary budget for the team. They busted a gut over the next couple of years delivering great work and over two pay cycles got to a place where they were making more than me. The way the hiring cycle worked out, they needed to pay more when they recruited me than when they recruited the manager a couple of years ago.
When I felt I was underpaid I made noises about looking to move, booked a couple of half days off and then came in wearing my best suit, and BOOM, out-of-sequence raise time. I'm still better paid per hour than my manager. My employer is not fair.
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I was also in the position where I was mentoring someone who was brand new to the company, and the industry, but was a couple of decades older than me and paid considerably more than me (and at a higher grade than me), being trained to do the exact same work as me. Although it grated to be training someone on basic things knowing they were being paid more, that was just the way it was. The company would have preferred to have hired a second person with my level of experience and at my wage, they weren't around, so they had to pay more. If I'd have made a fuss about this or asked to be moved up two grades and paid vastly more it would have looked like I didn't understand some basic things about how life works and that sometimes things aren't fair.
Do you know if you are underpaid for your work? Could you get a job elsewhere paid the same or more as this person? Do they have skills that you don't have and are being paid for that even as they "super-sub" into your team?