:orphan:
# PetscCommDuplicate
Duplicates the communicator only if it is not already a PETSc communicator.
## Synopsis
```
#include "petscsys.h"
PetscErrorCode PetscCommDuplicate(MPI_Comm comm_in, MPI_Comm *comm_out, PetscMPIInt *first_tag)
```
Collective
## Input Parameter
- ***comm_in -*** Input communicator
## Output Parameters
- ***comm_out -*** Output communicator. May be `comm_in`.
- ***first_tag -*** Tag available that has not already been used with this communicator (you may pass in `NULL` if you do not need a tag)
## Note
PETSc communicators are just regular MPI communicators that keep track of which
tags have been used to prevent tag conflict. If you pass a non-PETSc communicator into
a PETSc creation routine it will attach a private communicator for use in the objects communications.
The internal `MPI_Comm` is used to perform all the MPI calls for PETSc, the outer `MPI_Comm` is a user
level `MPI_Comm` that may be performing communication for the user or other library and so IS NOT used by PETSc.
## See Also
`PetscObjectGetNewTag()`, `PetscCommGetNewTag()`, `PetscCommDestroy()`
## Level
developer
## Location
src/sys/objects/tagm.c
## Examples
src/ksp/ksp/tutorials/ex73.c
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[Edit on GitLab](https://gitlab.com/petsc/petsc/-/edit/release/src/sys/objects/tagm.c)
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