Data import: accounts and contacts are being overwritten, not skipped

Data import: accounts and contacts are being overwritten, not skipped

Hello, I need urgent help - in the past I have successfully imported a CSV file, whether contacts or accounts, and any duplicate email addresses have been skipped.

Today both types of import are overwriting existing records, not skipping them.

At time of import there is no option for me to check saying "skip duplicates".

Please help me get this sorted?

Thanks in advance
Jo