What you most probably are trying to do, is loading a document with a docname, i.e. with an ino:docname attribute, that already exists. As of v3.1, docnames have to be unique.
So a general advise is to care for unique (ino:)docnames in the input file(s) for the Data Loader.
So if only 1 of your 150000 documents in the second input file has the same docname as one of the docs in the first inputfile, the second load will be completely rejected and rolled back (as you have experienced). As there is currently no way of guaranteeing uniqueness of docnames before the final stage of the Data Load, your better off to care for this particular point yourself.
This behaviour will be changed in a future version, so only the documents with the duplicate docnames are rejected.
All the best,
Hermann Gunde
#webMethods#Tamino#API-Management