I am having one custom object name as club with custom lookup field with contact object.
Let us assume we have 3 records in my club object(club1,club2 and club3).
When I assigned one member to club1 record , I shouldn't assign the same member to another record either in club2 or club3.
If I try to assign the same member to other club records I should be stopped with an error message.
How this can be achieved.
Thanks in advance