One way to delete unique entries will be to utilize a new column, using the formula below, to clearly label each entry as unique or not. Then you can filter the entire table, viewing only the unique values (as seen with a "TRUE" result from the formula below), and delete only those rows.
Put this in the top cell of a free column and fill it through the end of the column:
This assumes no header row, and that your data with dupes/unique values is in Column A.
Then you will want to turn filtering on for the column you just put the formula in. Ctrl+Shift+L in Excel 2013, or under the Data menu.
Note: There is a "6" in my cell A1
Then click the filter drop-down in the new TRUE/FALSE column and uncheck "FALSE" to show only uniques and click OK. Then Select the visible rows and delete those rows (right-click any row -> delete row).
Then just turn the filter back off (if it's still on) and you have all your dupes.