Inventory Sign Out Problem

[JJG]

I have an inventory database with 3 tables: a user table, and inventory table, and a transaction table. When a user checks out equipment it stores the in and out dates in the transaction table. How can I get it to not allow multiple check outs of the same item. For example, if there is no check in date in the transaction table for the item - it will not let you check it out because it is unavailable.

Each item has its own EquipmentID. I want a dialog box to pop up saying that the item is unavailable if it is selected for check out and already out.

Help!