So, the final recipe looks like following:

For Insert and Update calculate the hash e.g. with MD5 and put it in Hash property. Index it with not unique index.

Find duplicates with SQL query on Hash where group by for hash shows more than one occurrence.

Loop through all the values with same hash just and check Text property values to be sure that they are duplicates but not hash collisions.

Thanks, Alex, Dmitry, Robert!