Form "construction_budget" has a field that looks up from form "plans" this field is a formula field that concatenates the plans square feet and name. On the construction_budget form, this lookup field is named "plan_name"

I need field plan_name to exclude records from this field lookup if that record exists in the current forms records. 

That way only items that need a construction budget set, show up in the lookup, stopping duplicates from happening, as I can't make the lookup field be unique at all. And to make it easier for users to select the appropriate plan, as they should only have one plan to set a budget for if they added it to the master table form new_plans in the first place? Any thoughts on where to start with this? I am stuck.