diff --git a/pddl/definitions/constants_def.py b/pddl/definitions/constants_def.py
index 7ff8c1b..9f6032f 100644
--- a/pddl/definitions/constants_def.py
+++ b/pddl/definitions/constants_def.py
@@ -30,7 +30,9 @@ def __init__(
         constants: Optional[Collection[Constant]],
     ) -> None:
         """Initialize the PDDL constants section validator."""
-        TermsValidator(requirements, types).check_terms(constants if constants else [])
+        TermsValidator(requirements, types, no_duplicates=True).check_terms(
+            constants if constants else []
+        )
 
         super().__init__(requirements, types)
         self._constants = ensure_set(constants)
diff --git a/pddl/validation/terms.py b/pddl/validation/terms.py
index f2f5e37..2f03648 100644
--- a/pddl/validation/terms.py
+++ b/pddl/validation/terms.py
@@ -12,7 +12,16 @@
 
 """Module for validator of terms."""
 from functools import partial
-from typing import AbstractSet, Collection, Dict, Generator, Optional, Set, Type, Union
+from typing import (
+    AbstractSet,
+    Collection,
+    Dict,
+    Generator,
+    Mapping,
+    Optional,
+    Type,
+    Union,
+)
 
 from pddl.custom_types import name as name_type
 from pddl.definitions.base import TypesDef
@@ -35,12 +44,14 @@ def __init__(
         requirements: AbstractSet[Requirements],
         types: TypesDef,
         must_be_instances_of: Optional[Union[Type[Constant], Type[Variable]]] = None,
+        no_duplicates: bool = False,
     ):
         """Initialize the validator."""
         super().__init__(requirements, types)
 
         # if none, then we don't care if constant or variable
         self._allowed_superclass = must_be_instances_of
+        self._no_duplicates = no_duplicates
 
     def check_terms_consistency(self, terms: Collection[Term]):
         """
@@ -58,28 +69,41 @@ def _check_terms_consistency_iterator(
         Iterate over terms and check that terms with the same name must have the same type tags.
 
         In particular:
+        - if no_duplicates=Term there cannot be terms with the same name (variable or constant);
         - terms with the same name must be of the same term type (variable or constant);
         - terms with the same name must have the same type tags.
         """
-        seen: Dict[name_type, Set[name_type]] = {}
+        seen: Dict[name_type, Term] = {}
         for term in terms:
+            self._check_already_seen_term(term, seen)
             self._check_same_term_has_same_type_tags(term, seen)
             self._check_term_type(term, term_type=self._allowed_superclass)
             yield term
+            seen[term.name] = term
+
+    def _check_already_seen_term(self, term: Term, seen: Mapping[name_type, Term]):
+        """Check whether a term has been already seen earlier in the terms list."""
+        if self._no_duplicates and term.name in seen:
+            same_name_but_different_type = type(term) is not type(  # noqa: E721
+                seen[term.name]
+            )
+            check(
+                same_name_but_different_type,
+                f"Term '{term}' occurred twice in the same list of terms",
+                exception_cls=PDDLValidationError,
+            )
 
     @classmethod
     def _check_same_term_has_same_type_tags(
-        cls, term: Term, seen: Dict[name_type, Set[name_type]]
+        cls, term: Term, seen: Dict[name_type, Term]
     ) -> None:
         """
         Check if the term has already been seen and, if so, that it has the same type tags.
 
         This is an auxiliary method to simplify the implementation of '_check_terms_consistency_iterator'.
         """
-        if term.name not in seen:
-            seen[term.name] = set(term.type_tags)
-        else:
-            expected_type_tags = seen[term.name]
+        if term.name in seen:
+            expected_type_tags = seen[term.name].type_tags
             actual_type_tags = set(term.type_tags)
             check(
                 expected_type_tags == actual_type_tags,
diff --git a/tests/test_domain.py b/tests/test_domain.py
index d44a95d..b950bba 100644
--- a/tests/test_domain.py
+++ b/tests/test_domain.py
@@ -123,8 +123,8 @@ def test_constants_type_not_available() -> None:
         Domain("test", requirements={Requirements.TYPING}, constants={a}, types=type_set)  # type: ignore
 
 
-def test_constants_duplicates_with_different_types() -> None:
-    """Test that when two constants have same name but different types we raise error."""
+def test_constants_duplicates() -> None:
+    """Test that when two constants have same name we raise error."""
     a1 = Constant("a", type_tag="t1")
     a2 = Constant("a", type_tag="t2")
 
@@ -132,7 +132,7 @@ def test_constants_duplicates_with_different_types() -> None:
 
     with pytest.raises(
         PDDLValidationError,
-        match=r"Term a occurred twice with different type tags: previous type tags \['t1'\], new type tags \['t2'\]",
+        match=r"Term 'a' occurred twice in the same list of terms",
     ):
         Domain("test", requirements={Requirements.TYPING}, constants=[a1, a2], types=type_set)  # type: ignore