My code seems to be receiving particular messages more than once -- I can tell from the unique variables they contain. I have verified that each unique variable is being sent exactly once. The sender simply calls MPI_ISend to send a single message containing an array of variables. The receiver pseudo-code is as follows. Have I made a mistake in my use of MPI_Iprobe? MPI_Status status; int flag; // loop in case there was more than one message queued up due to asynchrony between // sender and receiver. Most of the time, there is only one message. do { flag = false; MPI_Iprobe (sender_rank, tag, intercom, &flag, &status); if (flag) { int len; MPI_Get_count (&status, data_type, &len); MPI_Recv (buffer, len, data_type, sender_rank, tag, intercom, MPI_STATUS_IGNORE); // at this point, sometimes I see duplicate variable values in the buffer, in different // calls to MPI_Recv. That is, s supposedly unique variable appears in two different // message. } } while(flag); Thanks for your help. Here is my build information: MPICH Version: 4.0.1 MPICH Release date: Tue Feb 22 16:37:51 CST 2022 MPICH Device: ch3:nemesis MPICH configure: --prefix=/opt/mpich --with-device=ch3:nemesis --with-slurm --disable-fortran -enable-debuginfo --enable-g=debug MPICH CC: gcc -g -O2 MPICH CXX: g++ -g -O2 MPICH F77: -g MPICH FC: -g