Compensate all of your employees so that every individual feels fairly treated. They will not worry, think, or be distracted by this topic, but instead will focus on work.
If you don't compensate an employee fairly, then you should assume that they might talk to each other, uncover disparities, and start becoming distracted. It is safest to fairly compensate an employee fair market value even when this is higher than they originally request. (Yes, I once had an employer give me more than I asked because I didn't understand the standard of living in that area... and I loved that job!)
WARNING: If you don't compensate MULTIPLE employees in a way they feel is fair, then I can almost guarantee you that they will not only be distracted by this, but they will start talking to each other. When people think they have a good deal, they are afraid to tell others in fear that they might lose that upper hand. But if they aren't sure, then it is in their best interest to find out. Nothing pulls a team of people together quicker than an important common cause, so this is a quick team building activity. Unfortunately, it is not one an employer should promote.
When people talk to each other about compensation (with a precondition of being unhappy), it is very likely they will come to conclusions that are not to your benefit (and possibly not even valid).
My advice... nip these issues in the bud as early as possible. The longer you let human minds ponder these issues, the more emotional (or even irrational) they are likely to become.