Report multiplying expressions by 4

This is not likely to be an issue with expressions. If I had to guess I'd say there is probably some issue with the data source and joins in your queries.

This is an overly simplistic example, but If you did something like join Orders to Customers using the Customer lastname field and you had 4 customers with a surname of "Smith" then and order for Smith would be repeated 4 times. So it could be a particular set of parameter values causing this issue