I want my list to have a default sort order that is based on multiple columns. For example:

order by
U.LastName,
U.FirstName

The list should be re-sortable by the user on any column. The trouble is, if I try to use SORTTAG like this:

order by
[SORTTAG, U.UserID],
U.LastName,
U.FirstName

... and then click the FirstName or LastName columns a SQL error appears:

System.Data.SqlClient.SqlException: A column has been specified more than once in the order by list.

How can I solve this?

When you're defining your {SORT} tags, you can place the default sort order in there. It's the second-to-last parameter. "" will leave it unsorted. "ASC" or "DESC" will sort on that column Ascending or Descending respectively. Initial sort order by the columns is determined by the sort indexer, the final parameter. Remember that the index is just a number, and it doesn't necessarily have to be entered in numerical order - that is, you may want to sort on LastName first, but display FirstName first. In that case, the index for LastName can still be 1, and FirstName 2 even if FirstName is displayed first.

Thanks! This is just what I needed.