Thus, singles and the last record of a set of duplicates are selected. That is, only truly duplicate records are selected. Both records are written to the primary output stream when their key fields are not equal. Records are read from the primary input stream. Secondary streams may be defined, but the secondary input stream must not be connected.
Output is written to the primary output stream and the secondary output stream depending on options.
An input record is written to exactly one output stream when both output streams are connected. It verifies that the sec- ondary input stream is not connected and then commits to level 0. It is normal to sort the file earlier in the pipeline.