diff --git a/Code/GraphMol/FileParsers/MolFileParser.cpp b/Code/GraphMol/FileParsers/MolFileParser.cpp
index d5642c578..8bd021a4b 100644
--- a/Code/GraphMol/FileParsers/MolFileParser.cpp
+++ b/Code/GraphMol/FileParsers/MolFileParser.cpp
@@ -245,7 +245,21 @@ std::string parseEnhancedStereo(std::istream *inStream, unsigned int &line,
       for (size_t i = 0; i < count; ++i) {
         ss >> index;
         // atoms are 1 indexed in molfiles
-        atoms.push_back(mol->getAtomWithIdx(index - 1));
+        auto atom = mol->getAtomWithIdx(index - 1);
+        if (std::ranges::find(atoms, atom) != atoms.end()) {
+          std::string message =
+              (boost::format(
+                   "Atom %1% appears more than once in stereo group specification on line %2%!") %
+               index % line)
+                  .str();
+          if (strictParsing) {
+            throw FileParseException(message);
+          } else {
+            BOOST_LOG(rdWarningLog) << message << std::endl;
+          }
+        } else {
+          atoms.push_back(atom);
+        }
       }
       std::vector<Bond *> newBonds;
       groups.emplace_back(grouptype, std::move(atoms), std::move(newBonds),
diff --git a/Code/GraphMol/FileParsers/file_parsers_catch.cpp b/Code/GraphMol/FileParsers/file_parsers_catch.cpp
index df94c4bf8..adce444f2 100644
--- a/Code/GraphMol/FileParsers/file_parsers_catch.cpp
+++ b/Code/GraphMol/FileParsers/file_parsers_catch.cpp
@@ -4107,7 +4107,9 @@ M  V30 1 1 1 2
 M  V30 END BOND
 M  V30 END CTAB
 M  END)CTAB";
-    { REQUIRE_THROWS_AS(MolBlockToMol(ctab), FileParseException); }
+    {
+      REQUIRE_THROWS_AS(MolBlockToMol(ctab), FileParseException);
+    }
     {
       bool sanitize = true;
       bool removeHs = true;
@@ -8059,4 +8061,48 @@ M  END
     }
     CHECK(foundPyrroleN);
   }
+}
+
+TEST_CASE("duplicates in stereo groups") {
+  auto ctab = R"CTAB(
+  Mrv1642508181718102D          
+
+  0  0  0     0  0            999 V3000
+M  V30 BEGIN CTAB
+M  V30 COUNTS 8 7 0 0 0
+M  V30 BEGIN ATOM
+M  V30 1 C -5.2222 4.7778 0 0 CFG=2
+M  V30 2 Br -6.9685 5.5478 0 0
+M  V30 3 C -5.0557 3.2468 0 0
+M  V30 4 C -3.9796 5.6874 0 0 CFG=2
+M  V30 5 C -2.5705 5.0661 0 0 CFG=1
+M  V30 6 F -1.3279 5.9758 0 0
+M  V30 7 C -4.1461 7.2184 0 0
+M  V30 8 C -2.404 3.5352 0 0
+M  V30 END ATOM
+M  V30 BEGIN BOND
+M  V30 1 1 1 2
+M  V30 2 1 1 4
+M  V30 3 1 4 5
+M  V30 4 1 5 6
+M  V30 5 1 4 7 CFG=1
+M  V30 6 1 5 8 CFG=1
+M  V30 7 1 1 3 CFG=1
+M  V30 END BOND
+M  V30 BEGIN COLLECTION
+M  V30 MDLV30/STEREL1 ATOMS=(2 4 4)
+M  V30 END COLLECTION
+M  V30 END CTAB
+M  END
+)CTAB";
+  // CHECK_THROWS_AS(v2::FileParsers::MolFromMolBlock(ctab),
+  // FileParseException);
+  v2::FileParsers::MolFileParserParams params;
+  params.strictParsing = false;
+  auto m = v2::FileParsers::MolFromMolBlock(ctab, params);
+  REQUIRE(m);
+  const auto &stgs = m->getStereoGroups();
+  REQUIRE(stgs.size() == 1);
+  CHECK(stgs.front().getGroupType() == StereoGroupType::STEREO_OR);
+  CHECK(stgs.front().getAtoms().size() == 1);
 }
\ No newline at end of file
diff --git a/Code/GraphMol/SmilesParse/CXSmilesOps.cpp b/Code/GraphMol/SmilesParse/CXSmilesOps.cpp
index ed460ce3c..143a3b40e 100644
--- a/Code/GraphMol/SmilesParse/CXSmilesOps.cpp
+++ b/Code/GraphMol/SmilesParse/CXSmilesOps.cpp
@@ -1364,6 +1364,13 @@ bool parse_enhanced_stereo(Iterator &first, Iterator last, RDKit::RWMol &mol,
               << "Atom " << aidx << " not found!" << std::endl;
           return false;
         }
+        if (std::ranges::find(atoms, atom) != atoms.end()) {
+          BOOST_LOG(rdWarningLog)
+              << "Atom " << aidx
+              << " appears more than once in stereo group specification!"
+              << std::endl;
+          return false;
+        }
         atoms.push_back(atom);
       }
     } else {
diff --git a/Code/GraphMol/SmilesParse/cxsmiles_test.cpp b/Code/GraphMol/SmilesParse/cxsmiles_test.cpp
index 635e41ea8..8819b23ba 100644
--- a/Code/GraphMol/SmilesParse/cxsmiles_test.cpp
+++ b/Code/GraphMol/SmilesParse/cxsmiles_test.cpp
@@ -1731,4 +1731,12 @@ TEST_CASE("atom maps and dummy labels in CXSMILES") {
     m->getAtomWithIdx(2)->setProp(common_properties::dummyLabel, "R1");
     CHECK(MolToCXSmiles(*m) == "CC[*:1] |atomProp:2.dummyLabel.R1|");
   }
+}
+
+TEST_CASE("duplicate atoms in StereoGroup") {
+  SECTION("as reported") {
+    std::string smiles = "C[C@H](F)[C@H](C)[C@@H](C)Br |a:1,o1:3,3|";
+
+    CHECK_THROWS_AS(SmilesToMol(smiles), SmilesParseException);
+  }
 }
\ No newline at end of file
diff --git a/Code/GraphMol/StereoGroup.cpp b/Code/GraphMol/StereoGroup.cpp
index 28038568e..5e04632e4 100644
--- a/Code/GraphMol/StereoGroup.cpp
+++ b/Code/GraphMol/StereoGroup.cpp
@@ -42,14 +42,24 @@ void assignMissingIds(const boost::dynamic_bitset<> &ids, unsigned &nextId,
     sg.setWriteId(nextId);
   }
 };
+
+template <typename T>
+void checkForDupes(const std::vector<T *> &vec, const std::string &typeName) {
+  for (auto it = vec.cbegin(); it != vec.cend(); ++it) {
+    if (std::find(vec.cbegin(), it, *it) != it) {
+      throw ValueErrorException("Duplicate " + typeName + " in StereoGroup");
+    }
+  }
+}
+
 }  // namespace
 
 StereoGroup::StereoGroup(StereoGroupType grouptype, std::vector<Atom *> &&atoms,
                          std::vector<Bond *> &&bonds, unsigned readId)
-    : d_grouptype(grouptype),
-      d_atoms(atoms),
-      d_bonds(bonds),
-      d_readId{readId} {}
+    : d_grouptype(grouptype), d_atoms(atoms), d_bonds(bonds), d_readId{readId} {
+  checkForDupes(d_atoms, "atom");
+  checkForDupes(d_bonds, "bond");
+}
 
 StereoGroup::StereoGroup(StereoGroupType grouptype,
                          const std::vector<Atom *> &atoms,
@@ -57,7 +67,10 @@ StereoGroup::StereoGroup(StereoGroupType grouptype,
     : d_grouptype(grouptype),
       d_atoms(std::move(atoms)),
       d_bonds(std::move(bonds)),
-      d_readId{readId} {}
+      d_readId{readId} {
+  checkForDupes(d_atoms, "atom");
+  checkForDupes(d_bonds, "bond");
+}
 
 StereoGroupType StereoGroup::getGroupType() const { return d_grouptype; }
 
diff --git a/Code/GraphMol/catch_graphmol.cpp b/Code/GraphMol/catch_graphmol.cpp
index ce4ac0269..524cfa9ab 100644
--- a/Code/GraphMol/catch_graphmol.cpp
+++ b/Code/GraphMol/catch_graphmol.cpp
@@ -1,5 +1,5 @@
 //
-//  Copyright (C) 2018-2024 Greg Landrum and other RDKit contributors
+//  Copyright (C) 2018-2026 Greg Landrum and other RDKit contributors
 //
 //   @@ All Rights Reserved @@
 //  This file is part of the RDKit.
@@ -5017,4 +5017,31 @@ TEST_CASE("canonical re-kekulization after sanitization preserves stereo",
     auto smi = MolToSmiles(*rwmol);
     CHECK(smi == refSmi);
   }
+}
+
+TEST_CASE("duplicate atoms/bonds in StereoGroups") {
+  SECTION("atoms") {
+    auto m = "C[C@H](O)C[C@H](F)Cl"_smiles;
+    REQUIRE(m);
+
+    std::unique_ptr<StereoGroup> stg;
+    CHECK_THROWS_AS(
+        stg = std::make_unique<StereoGroup>(
+            StereoGroupType::STEREO_ABSOLUTE,
+            std::vector<Atom *>{m->getAtomWithIdx(1), m->getAtomWithIdx(4),
+                                m->getAtomWithIdx(1)},
+            std::vector<Bond *>{}),
+        ValueErrorException);
+  }
+  SECTION("bonds") {
+    auto m = "C/C=C/c1ccc2ncccc2c1"_smiles;
+    REQUIRE(m);
+
+    std::unique_ptr<StereoGroup> stg;
+    CHECK_THROWS_AS(
+        stg = std::make_unique<StereoGroup>(
+            StereoGroupType::STEREO_ABSOLUTE, std::vector<Atom *>{},
+            std::vector<Bond *>{m->getBondWithIdx(1), m->getBondWithIdx(1)}),
+        ValueErrorException);
+  }
 }
\ No newline at end of file