diff --git a/src/ert/config/gen_kw_config.py b/src/ert/config/gen_kw_config.py
index 49302f41c37..8f8d5f4e746 100644
--- a/src/ert/config/gen_kw_config.py
+++ b/src/ert/config/gen_kw_config.py
@@ -157,7 +157,17 @@ def _check_non_negative_parameter(param: str, prior: PriorDict) -> None:
                     ).set_context(self.name)
                 )
 
+        unique_keys = set(str)
         for prior in self.get_priors():
+            key = prior["key"]
+            if key in unique_keys:
+                errors.append(
+                    ErrorInfo(
+                        f"Duplicate GEN_KW keys {key!r} found, keys must be unique."
+                    ).set_context(self.name)
+                )
+            unique_keys.add(key)
+
             if prior["function"] == "LOGNORMAL":
                 _check_non_negative_parameter("MEAN", prior)
                 _check_non_negative_parameter("STD", prior)
diff --git a/tests/unit_tests/config/test_gen_kw_config.py b/tests/unit_tests/config/test_gen_kw_config.py
index ef908ef4757..68e57c0dac3 100644
--- a/tests/unit_tests/config/test_gen_kw_config.py
+++ b/tests/unit_tests/config/test_gen_kw_config.py
@@ -34,6 +34,26 @@ def test_gen_kw_config():
     assert len(conf.transfer_functions) == 3
 
 
+@pytest.mark.usefixtures("use_tmpdir")
+def test_gen_kw_config_duplicate_keys_raises():
+    with pytest.raises(
+        ConfigValidationError,
+        match="Duplicate GEN_KW keys 'KEY2' found, keys must be unique.",
+    ):
+        GenKwConfig(
+            name="KEY",
+            forward_init=False,
+            template_file="",
+            transfer_function_definitions=[
+                "KEY1 UNIFORM 0 1",
+                "KEY2 UNIFORM 0 1",
+                "KEY2 UNIFORM 0 1",
+                "KEY3 UNIFORM 0 1",
+            ],
+            output_file="kw.txt",
+        )
+
+
 @pytest.mark.usefixtures("use_tmpdir")
 def test_gen_kw_config_get_priors():
     parameter_file = "parameters.txt"