1. A method of checking input customer information against existing information, the method comprising tolerating writing variations in input data by the steps of: determining in an input customer data record a value of a data field, the data field representing an identifier, determining, by a processor, from a reference data set of predetermined identifier values in a computer readable database at least one synonym candidate for the value of the data field using a candidate selection criterion, determining if a synonym candidate of the determined at least one synonym candidate and the value of the data field fulfill a predetermined synonym acceptance criterion based on at least one quality parameter, wherein said at least one quality parameter takes into account writing variations that are evaluated based on differences in the value of the data field and the synonym candidate, and when the predetermined synonym acceptance criterion is fulfilled, associating the value of the data field and the synonym candidate as synonyms and automatically updating a synonym set representing known writing variations for the identifier in a computer readable database and referencing to respective entries in the reference data set by adding the value of the data field to the synonym set as a member referring to the accepted synonym candidate in the reference data set without intervention of a user before searching for a counterpart for the input customer data record, and checking the input customer data record by searching for the counterpart for the input customer data record in the reference data set by comparing the value of the data field to the updated synonym set in the computer readable database after the step of determining if the predetermined synonym acceptance criterion is fulfilled.