diff --git a/src/cephadm/cephadm b/src/cephadm/cephadm
index c7f2b610433..09d1dce846e 100755
--- a/src/cephadm/cephadm
+++ b/src/cephadm/cephadm
@@ -2540,7 +2540,14 @@ def deploy_daemon(ctx, fsid, daemon_type, daemon_id, c, uid, gid,
 
     ports = ports or []
     if any([port_in_use(ctx, port) for port in ports]):
-        raise Error("TCP Port(s) '{}' required for {} already in use".format(','.join(map(str, ports)), daemon_type))
+        if daemon_type == 'mgr':
+            # non-fatal for mgr when we are in mgr_standby_modules=false, but we can't
+            # tell whether that is the case here.
+            logger.warning(
+                f"ceph-mgr TCP port(s) {','.join(map(str, ports))} already in use"
+            )
+        else:
+            raise Error("TCP Port(s) '{}' required for {} already in use".format(','.join(map(str, ports)), daemon_type))
 
     data_dir = get_data_dir(fsid, ctx.data_dir, daemon_type, daemon_id)
     if reconfig and not os.path.exists(data_dir):
diff --git a/src/pybind/mgr/cephadm/services/cephadmservice.py b/src/pybind/mgr/cephadm/services/cephadmservice.py
index f5ec2d908c4..12afdfbcd34 100644
--- a/src/pybind/mgr/cephadm/services/cephadmservice.py
+++ b/src/pybind/mgr/cephadm/services/cephadmservice.py
@@ -551,6 +551,18 @@ class MonService(CephService):
 class MgrService(CephService):
     TYPE = 'mgr'
 
+    def allow_colo(self) -> bool:
+        if self.mgr.get_ceph_option('mgr_standby_modules'):
+            # traditional mgr mode: standby daemons' modules listen on
+            # ports and redirect to the primary.  we must not schedule
+            # multiple mgrs on the same host or else ports will
+            # conflict.
+            return False
+        else:
+            # standby daemons do nothing, and therefore port conflicts
+            # are not a concern.
+            return True
+
     def prepare_create(self, daemon_spec: CephadmDaemonDeploySpec) -> CephadmDaemonDeploySpec:
         """
         Create a new manager instance on a host.