diff --git a/plumpy/exceptions.py b/plumpy/exceptions.py
index 40d3e12d..d7976b3a 100644
--- a/plumpy/exceptions.py
+++ b/plumpy/exceptions.py
@@ -33,3 +33,7 @@ class PersistenceError(Exception):
 
 class ClosedError(Exception):
     """Raised when an mutable operation is attempted on a closed process"""
+
+
+class DuplicateProcess(Exception):
+    """Raised when an ProcessLauncher is asked to launch a process it is already running."""
diff --git a/plumpy/process_comms.py b/plumpy/process_comms.py
index 3d9b6308..ecc75b95 100644
--- a/plumpy/process_comms.py
+++ b/plumpy/process_comms.py
@@ -4,12 +4,14 @@
 import copy
 import logging
 from typing import Any, cast, Dict, Optional, Sequence, TYPE_CHECKING, Union
+from weakref import WeakValueDictionary
 
 import kiwipy
 
-from . import loaders
+from . import exceptions
 from . import communications
 from . import futures
+from . import loaders
 from . import persistence
 from .utils import PID_TYPE
 
@@ -27,6 +29,7 @@
 
 if TYPE_CHECKING:
     from .processes import Process  # pylint: disable=cyclic-import
+    ProcessCacheType = WeakValueDictionary[PID_TYPE, Process]  # pylint: disable=unsubscriptable-object
 
 ProcessResult = Any
 ProcessStatus = Any
@@ -527,6 +530,20 @@ def __init__(
         else:
             self._loader = loaders.get_object_loader()
 
+        # using a weak reference ensures the processes can be garbage cleaned on completion
+        self._process_cache: 'ProcessCacheType' = WeakValueDictionary()
+
+    @property
+    def process_cache(self) -> 'ProcessCacheType':
+        """Return a dictionary mapping PIDs to launched processes that are still in memory.
+
+        The mapping uses a `WeakValueDictionary`, meaning that processes can be removed,
+        once they are no longer referenced anywhere else.
+        This means the dictionary will always contain all processes still running,
+        but potentially also processes that have terminated but have not yet been garbage collected.
+        """
+        return copy.copy(self._process_cache)
+
     async def __call__(self, communicator: kiwipy.Communicator, task: Dict[str, Any]) -> Union[PID_TYPE, ProcessResult]:
         """
         Receive a task.
@@ -571,15 +588,24 @@ async def _launch(
             init_kwargs = {}
 
         proc_class = self._loader.load_object(process_class)
-        proc = proc_class(*init_args, **init_kwargs)
+        proc: Process = proc_class(*init_args, **init_kwargs)
+
+        if proc.pid in self._process_cache and not self._process_cache[proc.pid].has_terminated():
+            raise exceptions.DuplicateProcess(f'Process<{proc.pid}> is already running')
+
         if persist and self._persister is not None:
             self._persister.save_checkpoint(proc)
 
+        self._process_cache[proc.pid] = proc
+
         if nowait:
             asyncio.ensure_future(proc.step_until_terminated())
             return proc.pid
 
-        await proc.step_until_terminated()
+        try:
+            await proc.step_until_terminated()
+        finally:
+            self._process_cache.pop(proc.pid, None)
 
         return proc.future().result()
 
@@ -602,15 +628,23 @@ async def _continue(
             LOGGER.warning('rejecting task: cannot continue process<%d> because no persister is available', pid)
             raise communications.TaskRejected('Cannot continue process, no persister')
 
+        if pid in self._process_cache and not self._process_cache[pid].has_terminated():
+            raise exceptions.DuplicateProcess(f'Process<{pid}> is already running')
+
         # Do not catch exceptions here, because if these operations fail, the continue task should except and bubble up
         saved_state = self._persister.load_checkpoint(pid, tag)
         proc = cast('Process', saved_state.unbundle(self._load_context))
 
+        self._process_cache[proc.pid] = proc
+
         if nowait:
             asyncio.ensure_future(proc.step_until_terminated())
             return proc.pid
 
-        await proc.step_until_terminated()
+        try:
+            await proc.step_until_terminated()
+        finally:
+            self._process_cache.pop(proc.pid, None)
 
         return proc.future().result()
 
diff --git a/test/rmq/test_communicator.py b/test/rmq/test_communicator.py
index a4e8f27b..2ca9b3a3 100644
--- a/test/rmq/test_communicator.py
+++ b/test/rmq/test_communicator.py
@@ -6,8 +6,8 @@
 import asyncio
 import shortuuid
 
+from kiwipy import BroadcastFilter, RemoteException, rmq
 import pytest
-from kiwipy import BroadcastFilter, rmq
 
 import plumpy
 from plumpy import communications, process_comms
@@ -204,3 +204,15 @@ async def test_continue(self, loop_communicator, async_controller, persister):
         # Let the process run to the end
         result = await async_controller.continue_process(pid)
         assert result, utils.DummyProcessWithOutput.EXPECTED_OUTPUTS
+
+    @pytest.mark.asyncio
+    async def test_duplicate_process(self, loop_communicator, async_controller, persister):
+        loop = asyncio.get_event_loop()
+        launcher = plumpy.ProcessLauncher(loop, persister=persister)
+        loop_communicator.add_task_subscriber(launcher)
+        process = utils.DummyProcessWithOutput()
+        persister.save_checkpoint(process)
+        launcher._process_cache[process.pid] = process
+        assert process.pid in launcher.process_cache
+        with pytest.raises(RemoteException, match='already running'):
+            await async_controller.continue_process(process.pid)