Keyboard problem: Always repeating 'r'

Let's visualize how a keyboard works as that might help you in isolating the issue.

Think how a spreadsheet has columns and rows so a given column line will intersect a given row this meeting of the two would be the firing of that key (shorting of the two lines) but what can happen here is you press on a different key the first is still shorted across so instead of two lines you have three or four shorted. Its also possible corrosion has shorted across the connections so it could be within the keyboard, cable connection or the logic board its self where the signals travel to then be decoded into the digital signal for the inner world of the computer from the outer world inputs.

OK now with your model info we can go a bit deeper.

So lets assume the keyboard got wet or a key is stuck then the easiest way to fix this is to replace the upper case assembly MacBook Air 13" (Mid 2013-2017) Upper Case with Keyboard and here's the guide MacBook Air 13" Early 2017 Upper Case Replacement

The other alternative is to replace the keyboard its self but its a fair amount of work! But a bit cheaper MacBook Air 13" (Mid 2011-Early 2015) Keyboard which also works in the 2017 models. But its a bear of a job! As you need to replace the plastic rivets with screws. So while being cheaper its a lot more work and does take a bit of practice to do well. You still need to follow the same guide to get to the keyboard removing carefully the backlight unit to gain access to the keyboard.

As you open the system carefully inspect your logic board for any corrosion around the keyboard connector and give the back side a very good inspection in case the wetness dripped down from the keyboard onto the logic board.