You might want to check your battery to make sure its not pressing up as that can happen. Most of the time I find the center keys get effected first. You'll need to remove it to check it out.
The other possibility is some keys get a beating as they tend to be used more. S's & R's are two of the most used letters.
Using the show keyboard menu option by enabling it with Keyboard Preferences will allow you to buy some time as well as a way to test your keyboards keys.
If you think it's not the battery, then it's likely time for a new keyboard. You can replace the full uppercase assembly or you can replace the keyboard alone. Here's the IFIXIT guide you'll need to follow: MacBook Pro 13" Unibody Mid 2012 Upper Case Replacement Here's the uppercase: MacBook Pro 13" Unibody (Early 2011-Mid 2012) Upper Case. Replacing the keyboard alone is a bit more work and you do need to be careful in taking the old one out. Frankly, I think just getting the uppercase is just easier.
Here's an image of a bad battery looks like:
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