diff --git a/src/ert/config/analysis_config.py b/src/ert/config/analysis_config.py
index 870c3676edb..9dd37fb59fc 100644
--- a/src/ert/config/analysis_config.py
+++ b/src/ert/config/analysis_config.py
@@ -195,7 +195,7 @@ def from_dict(cls, config_dict: ConfigDict) -> AnalysisConfig:
         if design_matrices:
             design_matrix = design_matrices[0]
             for dm_other in design_matrices[1:]:
-                design_matrix = design_matrix.merge_with_other(dm_other)
+                design_matrix.merge_with_other(dm_other)
         config = cls(
             max_runtime=config_dict.get(ConfigKeys.MAX_RUNTIME),
             minimum_required_realizations=min_realization,
diff --git a/src/ert/config/design_matrix.py b/src/ert/config/design_matrix.py
index da34bf37ca6..4d38933c2a6 100644
--- a/src/ert/config/design_matrix.py
+++ b/src/ert/config/design_matrix.py
@@ -78,7 +78,7 @@ def from_config_list(cls, config_list: list[str]) -> DesignMatrix:
             default_sheet=default_sheet,
         )
 
-    def merge_with_other(self, dm_other: DesignMatrix) -> DesignMatrix:
+    def merge_with_other(self, dm_other: DesignMatrix) -> None:
         errors = []
         if self.active_realizations != dm_other.active_realizations:
             errors.append(
@@ -110,8 +110,6 @@ def merge_with_other(self, dm_other: DesignMatrix) -> DesignMatrix:
         if errors:
             raise ConfigValidationError.from_collected(errors)
 
-        return self
-
     def merge_with_existing_parameters(
         self, existing_parameters: list[ParameterConfig]
     ) -> tuple[list[ParameterConfig], ParameterConfig | None]:
diff --git a/tests/ert/unit_tests/sensitivity_analysis/test_design_matrix.py b/tests/ert/unit_tests/sensitivity_analysis/test_design_matrix.py
index 3ed8c7309e2..b4992196ac7 100644
--- a/tests/ert/unit_tests/sensitivity_analysis/test_design_matrix.py
+++ b/tests/ert/unit_tests/sensitivity_analysis/test_design_matrix.py
@@ -6,6 +6,79 @@
 from ert.config.gen_kw_config import GenKwConfig, TransformFunctionDefinition
 
 
+def _create_design_matrix(xls_path, design_matrix_df, default_sheet_df) -> DesignMatrix:
+    with pd.ExcelWriter(xls_path) as xl_write:
+        design_matrix_df.to_excel(xl_write, index=False, sheet_name="DesignSheet01")
+        default_sheet_df.to_excel(
+            xl_write, index=False, sheet_name="DefaultValues", header=False
+        )
+    return DesignMatrix(xls_path, "DesignSheet01", "DefaultValues")
+
+
+@pytest.mark.parametrize(
+    "design_sheet_pd, default_sheet_pd, error_msg",
+    [
+        pytest.param(
+            pd.DataFrame(
+                {
+                    "REAL": [0, 1, 2],
+                    "c": [1, 2, 3],
+                    "d": [0, 2, 0],
+                }
+            ),
+            pd.DataFrame([["e", 1]]),
+            "",
+            id="ok_merge",
+        ),
+        pytest.param(
+            pd.DataFrame(
+                {
+                    "REAL": [0, 1, 2],
+                    "a": [1, 2, 3],
+                }
+            ),
+            pd.DataFrame([["e", 1]]),
+            "Design Matrices do not have unique keys",
+            id="not_unique_keys",
+        ),
+        pytest.param(
+            pd.DataFrame(
+                {
+                    "REAL": [0, 1],
+                    "d": [1, 2],
+                }
+            ),
+            pd.DataFrame([["e", 1]]),
+            "Design Matrices don't have the same active realizations!",
+            id="not_same_acitve_realizations",
+        ),
+    ],
+)
+def test_merge_multiple_occurrences(
+    tmp_path, design_sheet_pd, default_sheet_pd, error_msg
+):
+    design_matrix_1 = _create_design_matrix(
+        tmp_path / "design_matrix_1.xlsx",
+        pd.DataFrame(
+            {
+                "REAL": [0, 1, 2],
+                "a": [1, 2, 3],
+                "b": [0, 2, 0],
+            },
+        ),
+        pd.DataFrame([["a", 1], ["b", 4]]),
+    )
+
+    design_matrix_2 = _create_design_matrix(
+        tmp_path / "design_matrix_2.xlsx", design_sheet_pd, default_sheet_pd
+    )
+    if error_msg:
+        with pytest.raises(ValueError, match=error_msg):
+            design_matrix_1.merge_with_other(design_matrix_2)
+    else:
+        design_matrix_1.merge_with_other(design_matrix_2)
+
+
 @pytest.mark.parametrize(
     "parameters, error_msg",
     [