An index already exists for table <Tablename> with the columns <Columnnames>. (Visual Database Tools)
As a rule, you should create an index only if the data in the selected columns will be queried frequently. Indexes take up disk space and can slow the adding, deleting, and updating of rows.
In most cases, the benefits of indexes will far outweigh the performance overhead, but if your application updates data very frequently, or if you have disk space constraints, you might also want to limit the number of indexes.