A colleague of mine took up a second job, not because of money, but he (we) literally had nothing to do. Every day I’d ask my team lead, every day he’d say he’ll find some work for me. I thought it was unethical for my colleague to do two jobs, but is it though?

You know the irony? He didn’t have much to do even in his second job. So he was collecting two paychecks for doing almost nothing and was bored/irritated. Whose fault is that?

I think your effort and time would be better spent in making sure your employees had enough, interesting work to keep them occupied, instead of spending time playing detective. Let’s say you found one or more of your employees are doing multiple jobs. What can you do about it, other than getting upset and firing them? What happens after you fire them? You’re gonna hire again, and play detective again, with the new employees? It is just a waste of time.

Overworking your employees is bad. So is underworking. I’d rather spend time keeping my employees happy than snooping on them.