in the intended cases it means, that it was tried to add two different font attributes to the same font dictionary. But here is a case where this happens even without:

\pdfcompresslevel=0 \nopagenumbers \font\aa cmr10 at 10pt \pdffontattr\aa {foo} \aa abc \font\bb cmr10 at 11pt %\pdffontattr\bb {bar} \bb abc \bye

In the above case, I also encounter the warning. (Probably test files in vntex may be different.) Thanks Hartmut, Akira