A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
It seems there are several variables that we can investigate.
Somewhere along the line extra characters are being detected.
Your keyboard is hard-wire connected. That eliminates the possibility of problems such as low battery that can cause this problem in wireless keyboards.
If you turn off the second screen and use just one monitor, does that fix the problem?
If you boot to safe mode, does the problem go away?
macOS Sierra: Start up in safe mode - Apple Support
If you have a different keyboard, try plugging it in and see if that makes a difference.