At this point, you are trying to come up with a solution for which there is no problem. While it is possible that you and your seemingly marginal co-worker will get the same raise, that would really imply that management in your company is somewhere in the marginal zone, too.
Rather than focusing on a problem that may never exist, you will do far more for yourself by focusing on the realities of your job. You should be trying to do all in your power to maintain high levels of quality and productivity, along with positive working relationships with your fellow employees. If you get distracted with comparisons about who is getting what, you will be shortchanging yourself.
Now, if it should come to pass that you and your borderline associate receive the same raise, then you should discuss the raise policy with your manager. Without going into details about your co-worker, you can mention that you are confused over a policy that provides the same increases to everyone, regardless of performance. The next step is to listen carefully. Your manager's reaction is going to tell you a great deal about the way the company actually works. Then you have to decide if it works for you.
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