diff --git a/interfaces/cython/cantera/test/test_thermo.py b/interfaces/cython/cantera/test/test_thermo.py
index 0b4f1d1bc..384c5d618 100644
--- a/interfaces/cython/cantera/test/test_thermo.py
+++ b/interfaces/cython/cantera/test/test_thermo.py
@@ -133,6 +133,10 @@ class TestThermoPhase(utilities.CanteraTest):
             self.phase.X = 'H2:1e-1.4'
         self.assertArrayNear(X0, self.phase.X)
 
+        with self.assertRaises(Exception):
+            self.phase.X = 'H2:0.5, O2:1.0, H2:0.1'
+        self.assertArrayNear(X0, self.phase.X)
+
     def test_setCompositionDict(self):
         self.phase.X = {b'H2':1.0, b'O2':3.0}
         X = self.phase.X
diff --git a/src/base/stringUtils.cpp b/src/base/stringUtils.cpp
index 432e77b01..deb080eaf 100644
--- a/src/base/stringUtils.cpp
+++ b/src/base/stringUtils.cpp
@@ -18,6 +18,7 @@
 #include "cantera/base/stringUtils.h"
 #include "cantera/base/ctexceptions.h"
 #include "cantera/base/ctml.h"
+#include "cantera/base/utilities.h"
 
 #include <sstream>
 #include <cstdio>
@@ -155,6 +156,10 @@ compositionMap parseCompString(const std::string& ss,
             throw CanteraError("parseCompString",
                 "unknown species '" + name + "'");
         }
+        if (getValue(x, name, 0.0) != 0.0) {
+            throw CanteraError("parseCompString",
+                               "Duplicate key: '" + name + "'.");
+        }
         x[name] = fpValueCheck(ss.substr(valstart, stop-colon-1));
         start = ss.find_first_not_of(", ;\n\t", stop+1);
     }