A duplicate check based on the transactions ID (TX ID) is available here. The Software searches for duplicates according the same TX ID. The transaction ID is available in the trade list (full view) report under "manage columns".
If you have duplicate transactions that keep reoccurring due to duplicate data provided from the exchange, then they could be deleted or edited on the "Enter Coins" page and set all the values to zero except the date. This way the transaction remains in the system, but is not taken into account for calculations.
In addition you can set up your APIs and CSVs with a new start date so that the dates when duplicates appear are not imported anymore. It is described here in detail: How to avoid or unwanted transactions/re-imports or SCAM token via API jobs?