diff --git a/src/ert/config/design_matrix.py b/src/ert/config/design_matrix.py
index 641233631d5..e353e8efc32 100644
--- a/src/ert/config/design_matrix.py
+++ b/src/ert/config/design_matrix.py
@@ -5,6 +5,7 @@
 from typing import TYPE_CHECKING, List, Optional
 
 import pandas as pd
+from pandas.api.types import is_integer_dtype
 
 from ert.config.gen_kw_config import GenKwConfig, TransformFunctionDefinition
 
@@ -27,6 +28,8 @@ class DesignMatrix:
     xls_filename: Path
     design_sheet: str
     default_sheet: str
+    num_realizations: Optional[int] = None
+    active_realizations: Optional[list[int]] = None
     design_matrix_df: Optional[pd.DataFrame] = None
     parameter_configuration: Optional[dict[str, ParameterConfig]] = None
 
@@ -80,7 +83,13 @@ def read_design_matrix(
             self.xls_filename, self.design_sheet
         )
         if "REAL" in design_matrix_df.columns:
-            design_matrix_df = design_matrix_df.set_index("REAL", drop=True)
+            if not is_integer_dtype(design_matrix_df.dtypes["REAL"]) or any(
+                design_matrix_df["REAL"] < 0
+            ):
+                raise ValueError("REAL column must only contain positive integers")
+            design_matrix_df = design_matrix_df.set_index(
+                "REAL", drop=True, verify_integrity=True
+            )
         try:
             DesignMatrix._validate_design_matrix_header(design_matrix_df)
         except ValueError as err:
@@ -119,6 +128,10 @@ def read_design_matrix(
         design_matrix_df.columns = pd.MultiIndex.from_product(
             [[DESIGN_MATRIX_GROUP], design_matrix_df.columns]
         )
+        reals = design_matrix_df.index.tolist()
+        self.num_realizations = len(reals)
+        self.active_realizations = [x in reals for x in range(max(reals))]
+
         self.design_matrix_df = design_matrix_df
         self.parameter_configuration = parameter_configuration
 
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 d610a7f0fd3..145a3d43220 100644
--- a/tests/ert/unit_tests/sensitivity_analysis/test_design_matrix.py
+++ b/tests/ert/unit_tests/sensitivity_analysis/test_design_matrix.py
@@ -1,4 +1,6 @@
+import numpy as np
 import pandas as pd
+import pytest
 
 from ert.config import DesignMatrix
 
@@ -6,7 +8,73 @@
 def test_reading_design_matrix(tmp_path):
     design_path = tmp_path / "design_matrix.xlsx"
     design_matrix_df = pd.DataFrame(
-        {"REAL": [0, 1, 2], "a": [1, 2, 3], "b": [0, 2, 0], "c": [3, 1, 3]}
+        {
+            "REAL": [0, 1, 2],
+            "a": [1, 2, 3],
+            "b": [0, 2, 0],
+            "c": [3, 1, 3],
+        }
+    )
+    default_sheet_df = pd.DataFrame([["one", 1], ["b", 4], ["d", 6]])
+    with pd.ExcelWriter(design_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
+        )
+    design_matrix = DesignMatrix(design_path, "DesignSheet01", "DefaultValues")
+    design_matrix.read_design_matrix()
+
+
+@pytest.mark.parametrize(
+    "real_column, error_msg",
+    [
+        pytest.param([0, 1, 1], "Index has duplicate keys", id="duplicate entries"),
+        pytest.param(
+            [0, 1.1, 2],
+            "REAL column must only contain positive integers",
+            id="invalid float values",
+        ),
+        pytest.param(
+            [0, "a", 10],
+            "REAL column must only contain positive integers",
+            id="invalid types",
+        ),
+    ],
+)
+def test_reading_design_matrix_validate_reals(tmp_path, real_column, error_msg):
+    design_path = tmp_path / "design_matrix.xlsx"
+    design_matrix_df = pd.DataFrame(
+        {
+            "REAL": real_column,
+            "a": [1, 2, 3],
+            "b": [0, 2, 0],
+            "c": [3, 1, 3],
+        }
+    )
+    default_sheet_df = pd.DataFrame()
+    with pd.ExcelWriter(design_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
+        )
+    design_matrix = DesignMatrix(design_path, "DesignSheet01", "DefaultValues")
+    with pytest.raises(ValueError, match=error_msg):
+        design_matrix.read_design_matrix()
+
+
+def test_reading_design_matrix_duplicate_columns(tmp_path):
+    design_path = tmp_path / "design_matrix.xlsx"
+    design_matrix_df = pd.DataFrame(
+        {
+            "REAL": [0, 1, -4],
+            "a": [1, 2, 3],
+            "b": [0, 2, 0],
+            "c": [3, 1, 3],
+            "0": ["a", 2, "c"],
+        }
+    )
+    design_matrix_df = pd.DataFrame(
+        np.array([[1, 2, 3], [4, 5, 6], [7, 8, 9]]), columns=["a", "b", "a"]
     )
     default_sheet_df = pd.DataFrame([["one", 1], ["b", 4], ["d", 6]])
     with pd.ExcelWriter(design_path) as xl_write: