I believe the libjava Makefile.am on the gcj-eclipse branch has a bug with -j builds, where it may try to build the CNI header files multiple times.
Subject: Bug 29278 Author: tromey Date: Fri Oct 6 22:55:24 2006 New Revision: 117521 URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=117521 Log: PR libgcj/29278: * Makefile.in: Rebuilt. * Makefile.am ($(generic_header_files)): Depend on gcjh.stamp. (gcjh.stamp): New target. Modified: branches/gcj-eclipse/libjava/ChangeLog branches/gcj-eclipse/libjava/Makefile.am branches/gcj-eclipse/libjava/Makefile.in
Fix checked in.