The exact functionality you are looking is not directly built into LANDesk. While the LANDesk console uses SQL to run it's queries in the console, it is not as robust as an actual SQL server when it comes to writing queries. However, we can use the console to help us write the query and run it directly from a SQL server. Since you are trying to find devices that have the same service tag you can create a query in LANDesk like so (I chose Computer.BIOS.Service Tag but you can use whatever value you would like):
Once the query is created, right click the query in the console window and select "Inspect":
And then copy the SQL script into a SQL query window:
The whole point of creating the query and inspecting it is to find out exactly in the DB where the information you are trying to query off of is stored.
Once you have the SQL query, it is just a matter of modifying the query to meet your needs (add a Having Count clause to the statement).