diff --git a/custom_components/localthings/config_flow.py b/custom_components/localthings/config_flow.py
index c65fcec..d2a3608 100644
--- a/custom_components/localthings/config_flow.py
+++ b/custom_components/localthings/config_flow.py
@@ -215,8 +215,23 @@ def _is_placeholder_serial(serial: str) -> bool:
     serial` check here (and the equivalent one in coordinator.py's
     `_run_discovery`) doesn't catch it, and two such units get the same
     config-entry unique_id / entity unique_ids and collide (issue #83).
+
+    Issue #189: the DA_WM_A51_20_COMMON (ARTIK051) laundry board family
+    reports a flash-unset sentinel instead -- every character the same
+    repeated hex digit (a washer and a dryer, two different physical
+    units, both reported the literal serialNum 'FFFFFFFFFFFFFFF') -- which
+    the 'nothing' check above doesn't catch either, so the second unit's
+    config flow aborted as already configured.
     """
-    return serial.strip().lower().startswith('nothing')
+    s = serial.strip()
+    if s.lower().startswith('nothing'):
+        return True
+    upper = s.upper()
+    return (
+        len(upper) >= 8
+        and len(set(upper)) == 1
+        and upper[0] in '0123456789ABCDEF'
+    )
 
 
 def _probe_and_validate(host: str, ca_cert_pem: str, ca_key_pem: str) -> dict:
diff --git a/custom_components/localthings/coordinator.py b/custom_components/localthings/coordinator.py
index e895904..f17b1d8 100644
--- a/custom_components/localthings/coordinator.py
+++ b/custom_components/localthings/coordinator.py
@@ -92,12 +92,30 @@ def _is_placeholder_serial(serial: str) -> bool:
     serial` check below doesn't catch it, and `device_serial` feeds both
     the HA device-registry identifier and every entity's unique_id
     (entity.py), so two such units on the same install silently collide
-    and the second one's entities get dropped (issue #83). Mirrors the
-    identical helper in config_flow.py's `_probe_and_validate` -- kept
-    separate rather than imported to avoid pulling the config-flow module
-    into the runtime coordinator's import graph for a two-line check.
+    and the second one's entities get dropped (issue #83).
+
+    Issue #189: the DA_WM_A51_20_COMMON (ARTIK051) laundry board family
+    reports a flash-unset sentinel instead -- every character the same
+    repeated hex digit (a washer and a dryer, two different physical
+    units, both reported the literal serialNum 'FFFFFFFFFFFFFFF') -- which
+    the 'nothing' check above doesn't catch either, so two such units
+    collided on the config-entry unique_id and the second couldn't be
+    added at all.
+
+    Mirrors the identical helper in config_flow.py's `_probe_and_validate`
+    -- kept separate rather than imported to avoid pulling the config-flow
+    module into the runtime coordinator's import graph for a two-line
+    check.
     """
-    return serial.strip().lower().startswith('nothing')
+    s = serial.strip()
+    if s.lower().startswith('nothing'):
+        return True
+    upper = s.upper()
+    return (
+        len(upper) >= 8
+        and len(set(upper)) == 1
+        and upper[0] in '0123456789ABCDEF'
+    )
 
 
 class LocalThingsCoordinator(DataUpdateCoordinator[dict[str, Any]]):
diff --git a/tests/localthings/test_config_flow.py b/tests/localthings/test_config_flow.py
index 8d40bd3..492614d 100644
--- a/tests/localthings/test_config_flow.py
+++ b/tests/localthings/test_config_flow.py
@@ -539,3 +539,18 @@ def test_is_placeholder_serial_accepts_real_serials():
     from custom_components.localthings.config_flow import _is_placeholder_serial
     assert _is_placeholder_serial('0A1B2C3D4E5F') is False
     assert _is_placeholder_serial('') is False
+
+
+def test_is_placeholder_serial_catches_all_same_hex_digit():
+    """Issue #189: the DA_WM_A51_20_COMMON (ARTIK051) laundry board family
+    reports a flash-unset sentinel instead of 'Nothing(SVC)' -- every
+    character the same repeated hex digit. A washer and a dryer, two
+    different physical units, both reported the literal serialNum
+    'FFFFFFFFFFFFFFF', colliding on the config-entry unique_id."""
+    from custom_components.localthings.config_flow import _is_placeholder_serial
+    assert _is_placeholder_serial('FFFFFFFFFFFFFFF') is True
+    assert _is_placeholder_serial('ffffffffffffffff') is True
+    assert _is_placeholder_serial('00000000') is True
+    # Too short to be the flash-unset sentinel -- a real serial could
+    # plausibly repeat one hex digit seven times by chance.
+    assert _is_placeholder_serial('FFFFFFF') is False
diff --git a/tests/localthings/test_coordinator.py b/tests/localthings/test_coordinator.py
index e9e6f45..3b89d5e 100644
--- a/tests/localthings/test_coordinator.py
+++ b/tests/localthings/test_coordinator.py
@@ -15,7 +15,7 @@ from custom_components.localthings.const import (
     SUMMARY_INTERVAL_S,
 )
 from custom_components.localthings.coordinator import (
-    LocalThingsCoordinator, _local_source_port,
+    LocalThingsCoordinator, _is_placeholder_serial, _local_source_port,
 )
 from custom_components.localthings.registry.capabilities.common import (
     remote_control_enabled,
@@ -163,6 +163,44 @@ def test_run_discovery_falls_back_to_host_for_placeholder_serial(
     assert coordinator.device_serial == mock_entry.data[CONF_HOST]
 
 
+def test_run_discovery_falls_back_to_host_for_all_f_placeholder_serial(
+    hass: HomeAssistant, mock_entry
+) -> None:
+    """Issue #189: the DA_WM_A51_20_COMMON (ARTIK051) laundry board family
+    reports a flash-unset sentinel instead of 'Nothing(SVC)' -- every
+    character the same repeated hex digit. A washer and a dryer, two
+    different physical units, both reported the literal serialNum
+    'FFFFFFFFFFFFFFF', so without this fallback they'd collide on
+    device_serial exactly like the #83 case above."""
+    resources = {
+        '/information/vs/0': {
+            'x.com.samsung.da.modelNum':
+                'DA_WM_A51_20_COMMON|20221341|30010102001211000103000000000000',
+            'x.com.samsung.da.description':
+                'DA_WM_A51_20_COMMON_DVE50A8800/DC92-02835A_0080',
+            'x.com.samsung.da.serialNum': 'FFFFFFFFFFFFFFF',
+        },
+        '/otninformation/vs/0': {},
+    }
+    coordinator = LocalThingsCoordinator(hass, mock_entry)
+    coordinator._run_discovery(resources)
+    assert coordinator.device_serial == mock_entry.data[CONF_HOST]
+
+
+def test_is_placeholder_serial_catches_all_same_hex_digit():
+    assert _is_placeholder_serial('FFFFFFFFFFFFFFF') is True
+    assert _is_placeholder_serial('ffffffffffffffff') is True
+    assert _is_placeholder_serial('00000000') is True
+
+
+def test_is_placeholder_serial_accepts_real_serials_and_short_runs():
+    assert _is_placeholder_serial('0A1B2C3D4E5F') is False
+    assert _is_placeholder_serial('') is False
+    # Too short to be the flash-unset sentinel -- a real serial could
+    # plausibly repeat one hex digit seven times by chance.
+    assert _is_placeholder_serial('FFFFFFF') is False
+
+
 def test_run_discovery_detects_cooktop_via_resource_signature(
     hass: HomeAssistant, mock_entry
 ) -> None: