What you are doing wrong is selecting random numbers from a fixed range (example 1 to 10, always).
You need to change that range every time a number is selected to create a “pool” of possible numbers. Example : Pool is 1,2,3,4,5,6,7,8,9,10. If 4 is selected, then remove it from the pool and next time it will not be possible to select it again.
We all showed you up there how to do that. You will have to adapt it to your project.