If you see multiple rows for the same students or teachers, it is not an error. This is happening by design in the report builder when you select a piece of data for which the same student or teacher has multiple "answers," at which point a "multiple row" element is introduced. Basically, the report is displaying all applicable data, though you may decrease this to 1 row per student or teacher if you wish.
For example, suppose your report displays Student ID, 2011 ELA CST Scale Score, and Teacher. A student has 1 student ID and 1 scale score from that particular test, so ordinarily he would display as only 1 row in your report, but if the same student has had multiple teachers over time you will see a row for each teacher.
Where to Start

1. Click on the Reports tab.
2. Click View Reports (Prebuilt & Custom Reports).
3. Select the title of the report you would like to edit.
Pick Criteria for Filtering Data

Under Add Columns select:
1. Data Type - Core Data
2. Categories - Class Rosters
3. Columns - Class Rosters: Roster is Current?
4. Add Columns to Report.
Filter
1. Go to Filters
2. Click the green + sign to add a filter.
3. Select column Rosters Is Current?
4. Select is true.
5. Click Save Filters.
Next Steps
In some cases filtering will not be enough to remove duplicates. For an alternative, check out our lesson on Grouping.