Database normalization, should I combine these two tables? I am sort of new to database normalization and working with a database that I did not create. The database has a table for users, and a separate table for user addresses. Both tables use the user id as their primary key, and every user is required to have an address. Since every user is required to have an address, should the information be added to the user table for proper normalization?
At the theoretical level, it does sound like these two tables can be combined. 

At the practical level, unless you're rewriting everything that interacts with the database, no explicit significant harm should arise if you leave it as is. 

If the only duplication of data is the user id, then the space to be gained from the change is minor, and probably not worth the cost of modifying your existing code to find the data in its new location.

If we knew more of the specifics of your situation, we might be able to provide a more detailed answer.