Question: One of the rear rack holes on my aluminum frame have become stripped after a year of taking the rear rack off at least once a week. I've been using this bike for commuting and weekend riding, so I like to remove the rack when I'm not carrying panniers.

I've asked a few bike shops about rethreading the hole and have gotten multiple responses. One shop there is a tool that some shops have that can rethread it. Another said I was out of luck. One criticized me for taking the rack off often. The last one I asked said some shops could do it but you'd have to go up a screw size.

What's the consensus here on this type of frame repair? At the moment I can still use the rack since it's still secure with the one loose screw. I intend on getting a new bike at the end of the year for non-commuting, so I will make this a dedicated commuter bike where I can leave the rack on it when that happens.

Answer: 
Most anchor points in aluminum frames are not an actual tapped holes in the frame. It is a threaded inset that attaches to the frame similar to the way a pop rivet works. On older frames they were sometimes referred to as braze-ons. If you search for Nutsert or rivet nut they may be available at your local home center. While the individual inserts are not expensive the insertion tool can be close to $80. The inserts can also be troublesome to remove. Attempting to drill them out usually results in the insert spinning in the mounting hole. A local custom frame builder may be able to help you as they likely have the tool to install bottle cage mounts and such.
Going up a size (to M6) or installing a Helicoil are both good options if there's a reasonable amount of material surrounding the eyelet, which is true for all but the daintiest. If you want to keep taking it off regularly, the Helicoil is a little better option in terms of preventing similar issues in the future, since now you'll be doing it with steel threads. But either approach is sound and any real shop can do either as needed. 

The simple, non-shop fix is use a longer bolt plus a nut. If clearance is tight, use a buttonhead M5 bolt run from the inside out, so the nut is outside.