DB/Reporting :: Relation DB Design?

I am designing a system for our sales team to enter their sales expenses. I have designed an input form and need some advise on what exactly I should store in the expenses table VS lookup from the employee table.

First, the form has a text box where the user must enter their EmployeeID. Management asked for a text box VS a combo box because they do not want users looking at each others expenses. The form has a search feature where by after you enter your employee number, it will look for an existing expense ticket first before allowing you to create a new one. Anyway, here are some of the decisions I have to make.

When the user enters their employee number, I would like to display their employee name. The EmployeeID is saved in the Expense table but should I also store the name in the expsense table? If I do save it, it's duplication of data, if I do not, I have to look it up each time the record is loaded. But is there a point storing the name long term? Does this qualify as related data? If I was to perform some reporting, could this cause a problem if an employee changes their name etc.

The expense data is spread over 2 tables, the first for the header details including employeeID, expense date and Expense ID etc. The second table contains the expense lines information. Amount, customer, category etc. My next question is when the employee chooses an expense category, there is an associated general Ledger code with that category. Again, the expense category data is stored in a table. When the user chooses the expense category, should I store the categoryID or store the values associated with the ID (GLAcct, CategoryDescription etc.) Does this qualify as related data or should lookup values always be duplicated in the master table? It is possible that over time, the GL accounts associated with the Expense categories could change. So for accuracy purposes, should they be stored in the expense lines table?