For an agreement you can define 2 periods which overlaps. This should not be allowed, and koha should have a way to detect the "current" period (and ideally display separately)
Should it be a warning or really a hard block? I am wondering if there isn't a use case where an agreement might overlap.
Not sure... By the way the user story "Identify the "Current" Agreement period" was integrated into Taiga in 2022 : https://tree.taiga.io/project/joubu-koha-erm/us/11?kanban-status=2674133