If you want the process to not continue and give an error message when the object you selected in the reference selector is selected in the next reference selector, you can provide this with count. For example, you have a total of 5 objects to must to choose from, and when you save, you should have 5 objects in your list. However, by making this count action, if there are fewer objects in it (which means the same object has been selected more than once), you can get an error message and have it selected again. I have set up a system like this for you.
I hope it works,
It is impossible to select the same object over the same association twice.
No need to validate this.