I am taking data from 2 tables employees and inventory to make 3rd with equipment assignments.Now how to move data from that 3rd table once someone turn certain peace of equipment.Could I disable duplicate data and some kinda of warning that I am about to overwrite existing data or there is smarter way to solve this.( If my idea is good how to do that?)

For history purposes, you might want to leave the data there. You could have 2 date fields signifying the dates they took out and brought back the equipment. You could also add a yes/no field that got checked when they brought it back. Either way gives you a method to check if a particular piece of equipment is already out or not.