How could I define duplicate forms (I mean is it possible that my forms have been submitted more than once).
Do you mean you wish to know if your survey form has been submitted more than once?
Kindly please be informed that KoBoToolbox at the moment does not support this feature. Maybe, you could make a new KoBoToolbox feature’s request here if you wish to see this in the future.
As a workaround, you could be cautious while collecting data (where enumerators are allocated with specific respondents and they collect data from the same i.e. not going out of the samples they have been provided). The other option is during data management (where the data manager cleans the repeated samples if there are any looking at key identifiers such as name etc. set while developing the survey tool).