If you create a summary table grouped by the column you want to deduplicate, then sort the summary table by descending count, all the duplicated values will be at the top.
You can then use linking to select the original table by the summary table, meaning that clicking a row in the summary table will show only the corresponding rows in the original table.
Automatically removing duplicates requires defining which rows get removed. What do you want to happen when you import data that already exists in the table? Keep the new row from the import (can you assume there’s only one?) or the existing one in Grist?