Wrong letters when I'm typing with Macbook Pro

This unicode combination can be produced when holding down the alt/option key on your Mac's keyboard. If you google this issue, there is a thread from 2013/14 describing a solution to this issue.

Here are the summarised steps to revert they keyboard back to its intended use:

1. Open system preferences
2. Select accessibility
3. Select keyboard
4. Enable sticky keys
5. Press option twice quickly to enable sticky and once more to turn it off (option key must be enabled if you previously disabled it in modifier key controls)
6. Problem solved! - Turn sticky keys off again

I can verify that these steps are still valid for macOS High Sierra but do let me know how you get on.

If the sticky keys fix does not work, then the fact that an external keyboard works ok most likely indicates that the internal one is damaged.

Though this solution looks weird, but it works:

Run your fingers over all the keys. I too faced the same error because some key was stuck, rubbing fingers over all the keys helped me.