I'm trying to create a "team" using several look up fields and I need to create a filter/condition that prevents from the same lead user from being assigned more than once. Any advice on how to achieve this?
I'm not too clear what you are trying to do. But if you have fields "LookupA, LookupB, LookupC and LookupD" on your object to make the team, and each of them look up to Lead, I believe you can make a validation rule in core salesforce that there cannot be duplicates in your lookup fields. If this is not right - give me some more details about how you are building your team.