I disagree with this wholeheartedly. If you are constantly doing the work of multiple devs, you get zero reward in this industry.

Instead, put that extra focus into side work and side projects. If you’re good enough you can even be honest with people. When taking on a contract you can say “I have another commitment and will be working between X and X on X days for you” as long as you get all the work done for your primary job it’s not immoral.