diff --git a/changelog.md b/changelog.md
index f7f1ab35..d5cabb27 100644
--- a/changelog.md
+++ b/changelog.md
@@ -5,6 +5,8 @@
 > [!IMPORTANT]
 > This version is not released yet and is under active development.
 
+- Deduplicate platforms on `Group` instantiation.
+
 ## [1.4.0 (2024-10-21)](https://github.com/kdeldycke/extra-platforms/compare/v1.3.1...v1.4.0)
 
 - Allow set comparison between groups and single platform.
diff --git a/extra_platforms/groups.py b/extra_platforms/groups.py
index 528508e0..24651be4 100644
--- a/extra_platforms/groups.py
+++ b/extra_platforms/groups.py
@@ -96,19 +96,20 @@ class Group:
     """
 
     def __post_init__(self):
-        """Keep the platforms sorted by IDs."""
+        """Deduplicate platforms and sort them by IDs."""
         object.__setattr__(
             self,
             "platforms",
-            tuple(sorted(self.platforms, key=lambda p: p.id)),
+            tuple(sorted(set(self.platforms), key=lambda p: p.id)),
         )
         object.__setattr__(
             self,
             "platform_ids",
             frozenset({p.id for p in self.platforms}),
         )
-        # Double-check there is no duplicate platforms.
-        assert len(self.platforms) == len(self.platform_ids)
+        assert len(self.platforms) == len(
+            self.platform_ids
+        ), "The group contain platforms with duplicate IDs"
 
     def __iter__(self) -> Iterator[Platform]:
         """Iterate over the platforms of the group."""
diff --git a/tests/test_groups.py b/tests/test_groups.py
index 6af0e77e..e5255960 100644
--- a/tests/test_groups.py
+++ b/tests/test_groups.py
@@ -86,6 +86,11 @@
 from extra_platforms import groups as groups_module
 
 
+def test_platform_deduplication():
+    my_group = Group("my_group", "My Group", "✅", (AIX, AIX))
+    assert len(my_group) == 1
+
+
 def test_group_definitions():
     for group in ALL_GROUPS:
         # ID.