The CSV import extension will allow you to merge data from your import file to existing records in your base in order to prevent duplicate entries. However, if your file has duplicate entries within it (for example, five rows all with the same booking number), this will not be recognized via the merge feature as it only looks to records within your base to match against. To get around this, feel free to import the data and then use the dedupe extension to find and merge the duplicate records.
