Having 2 instances after deploy

I am a student learning about cloud computing and I am doing my project.
I don’t know what is the difference between Heroku and Render. But I don’t have this issue when I run my code on Heroku. However, we are required to use platform other than Heroku so I moved to Render. I have tried many platform but Render is the best so far, but I cannot fix the two instances problem.