Removing duplicates in Excel is easily done on the Data tab. Select Remove Duplicates and check only the box that corresponds to the column for which you want to remove duplicates. In the case below it is "Keyword".
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You can also view the entire competitive landscape for a term in one row, so you can see how you compete against all the competing websites you've analyzed. Add all the terms to a single sheet, and make sure each website's current position is in its own column.
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By now, you should be using an Excel PivotTable. If you don't know how to use PivotTables, here's a quick overview, but you can also read this tutorial or this one by Microsoft for a more detailed step-by-step guide.
I can't believe I worked at SEP for so many years without knowing how to do this. It will literally save you hundreds of hours. Create a pivot table in a new spreadsheet in your Excel file by selecting "Insert" and then "Pivot Table."
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Then select all the columns you want to merge from your other spreadsheet.
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When you create your pivot table, make sure your Rows are your Keywords (you can drag the keywords field into the rows area). Drag the other fields into the Values area and make sure to set metrics like search volume, keyword difficulty, CPC to something like the maximum value rather than the sum of the values - this will automatically add all the data for each keyword into a single row.