To get to the point at once I contacted the support and they gave me a hand. This is the feedback that Eric Hansen provided:
"I found a few postings in our knowledgebase that discuss how to handle duplicate records. In brief, once you have several records in QuickBase, you can self check for duplicates by creating report and then sorting and grouping by fields that should have unique values. If no one field is unique but a summary of 3 fields would help make it unique, you could create a summary field that combines the values of any 3 fields from your record and this field can then be reported on to find duplicates. (This field could even be a hidden field of course.)
I am sorry however at this time there is not an automated way to prevent or remove duplicate records built in to QuickBase."
So I decided to check it in practice applying to my app with 20 000 records. Having added formula field Name in which I united First and Last columns through blank space and built a summary report grouped by Name. This is what I have got:
Unfortunately Quickbase doent provide any means to eliminate rows from report where Number Of Customers = 1. By pressing Name we open records which have the same name and only then for each record it is possible to manually decide what to do.
It is possible to find duplicates in Quickbase, but it’s a pity you can’t eliminate unique records from report. For the rest you can be confident you’ll manage to handle it. There is no problem to delete a record. But the thing is it’s a problem to merge all details and to move them to one needed record. It can be accomplished manually of course, but this is the difficult part. Quickbase has no means to help out here.