From 5fe82fc485425569e57cdd15639379e5c254efba Mon Sep 17 00:00:00 2001
From: Vecko <36369090+VeckoTheGecko@users.noreply.github.com>
Date: Wed, 29 Apr 2026 15:39:25 +0200
Subject: [PATCH 1/4] Fix 2590

---
 src/parcels/_core/xgrid.py | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/src/parcels/_core/xgrid.py b/src/parcels/_core/xgrid.py
index efa69dce9..3bc4242c2 100644
--- a/src/parcels/_core/xgrid.py
+++ b/src/parcels/_core/xgrid.py
@@ -52,6 +52,19 @@ def assert_all_field_dims_have_axis(da: xr.DataArray, xgcm_grid: xgcm.Grid) -> N
                 f'HINT: You may want to add an {{"axis": A}} to your DataSet["{dim[1]}"], where A is one of "X", "Y", "Z" or "T"'
             )
 
+    ax_dims = cast(dict[str, str], ax_dims)
+
+    seen_axes: dict[str, str] = {}
+    for ax, dim_name in ax_dims:
+        if ax in seen_axes:
+            raise ValueError(
+                f"Two dimensions ({dim_name!r} and {seen_axes[ax]!r}) provide values in the axis direction {ax!r}. "
+                "This is not possible, a field cannot have two dimensions on a single axis."
+            )
+        seen_axes[ax] = dim_name
+    assert len(ax_dims) <= 4
+    return
+
 
 def _transpose_xfield_data_to_tzyx(da: xr.DataArray, xgcm_grid: xgcm.Grid) -> xr.DataArray:
     """

From 628efe245a9830b361a481313c36d658faf53bdd Mon Sep 17 00:00:00 2001
From: Vecko <36369090+VeckoTheGecko@users.noreply.github.com>
Date: Thu, 30 Apr 2026 13:40:42 +0200
Subject: [PATCH 2/4] Add test_dim_with_duplicate_axis

---
 tests/test_xgrid.py | 27 +++++++++++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

diff --git a/tests/test_xgrid.py b/tests/test_xgrid.py
index 0029e6328..53b0124e8 100644
--- a/tests/test_xgrid.py
+++ b/tests/test_xgrid.py
@@ -6,7 +6,7 @@
 import xarray as xr
 from numpy.testing import assert_allclose
 
-from parcels import Field
+from parcels import Field, FieldSet
 from parcels._core.index_search import (
     LEFT_OUT_OF_BOUNDS,
     RIGHT_OUT_OF_BOUNDS,
@@ -17,7 +17,7 @@
     XGrid,
     _transpose_xfield_data_to_tzyx,
 )
-from parcels._datasets.structured.generic import X, Y, Z, datasets
+from parcels._datasets.structured.generic import X, Y, Z, datasets, datasets_sgrid
 from parcels.interpolators import XLinear
 from tests import utils
 
@@ -151,6 +151,29 @@ def test_dim_without_axis():
         Field("z1d", ds["z1d"], grid, XLinear)
 
 
+def test_dim_with_duplicate_axis():
+    ds = datasets_sgrid["ds_2d_padded_low"].copy()
+
+    # Add an extra Z axis
+    ds = ds[["data_g", "grid"]]
+    ds["data_g"] = ds["data_g"].expand_dims("vertical_dimensions_dim2", 1)
+
+    z = ds["vertical_dimensions_dim2"]
+    z.attrs.update({"axis": "Z", "c_grid_axis_shift": 0})
+    ds["vertical_dimensions_dim2"] = z
+
+    # TODO: Clean up this attribute setting (really, this should be on the source datasets)
+    lon = ds["lon"]
+    lat = ds["lat"]
+    lon.attrs.update({"units": "metres"})
+    lat.attrs.update({"units": "metres"})
+    ds["lon"] = lon
+    ds["lat"] = lat
+
+    with pytest.raises(ValueError, match="Two dimensions .*provide values in the axis direction 'Z'."):
+        FieldSet.from_sgrid_conventions(ds)
+
+
 def test_vertical1D_field():
     nz = 11
     ds = xr.Dataset(

From c927fa70cee349f6de638dca88dddecc4e99ffab Mon Sep 17 00:00:00 2001
From: Vecko <36369090+VeckoTheGecko@users.noreply.github.com>
Date: Mon, 4 May 2026 14:48:23 +0200
Subject: [PATCH 3/4] Update assert statement

---
 src/parcels/_core/xgrid.py | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/parcels/_core/xgrid.py b/src/parcels/_core/xgrid.py
index 3bc4242c2..ff4195de4 100644
--- a/src/parcels/_core/xgrid.py
+++ b/src/parcels/_core/xgrid.py
@@ -62,7 +62,9 @@ def assert_all_field_dims_have_axis(da: xr.DataArray, xgcm_grid: xgcm.Grid) -> N
                 "This is not possible, a field cannot have two dimensions on a single axis."
             )
         seen_axes[ax] = dim_name
-    assert len(ax_dims) <= 4
+    assert len(ax_dims) <= 4, (
+        "Execution should never reach this point. Please file an issue sharing more about your input dataset."
+    )
     return
 
 

From af73d52c4f39b9edd37e03fb72172c3d50d6ea77 Mon Sep 17 00:00:00 2001
From: Nick Hodgskin <36369090+VeckoTheGecko@users.noreply.github.com>
Date: Mon, 4 May 2026 16:07:26 +0200
Subject: [PATCH 4/4] Update src/parcels/_core/xgrid.py

Co-authored-by: Erik van Sebille <e.vansebille@uu.nl>
---
 src/parcels/_core/xgrid.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/parcels/_core/xgrid.py b/src/parcels/_core/xgrid.py
index ff4195de4..3edd0875c 100644
--- a/src/parcels/_core/xgrid.py
+++ b/src/parcels/_core/xgrid.py
@@ -63,7 +63,7 @@ def assert_all_field_dims_have_axis(da: xr.DataArray, xgcm_grid: xgcm.Grid) -> N
             )
         seen_axes[ax] = dim_name
     assert len(ax_dims) <= 4, (
-        "Execution should never reach this point. Please file an issue sharing more about your input dataset."
+        "The input dataset appears to have more than 4 dimensions after conversion. Execution should never reach this point. Please file an issue sharing more about your input dataset."
     )
     return