Find rows where one column has multiple matching values in another column

Experts,

I have a products table in our MS SQL Server db.

ID, productID, productname, catalogID

The business rule we are going to start using says that each catalogID can have one and only one matching productID.

One productID can have the same catalogID, but not the other way around.

Eventually we will create separate tables with foreign keys, but for now all the columns are in the same table.

How would I find those catalogIds that have more than one matching productId?

For some reason I can't get my head around this.

Thanks in advance.