Fix check for duplicate method descriptor

Author: Zeavee

substratevm/src/com.oracle.svm.hosted/src/com/oracle/svm/hosted/imagelayer/SVMImageLayerWriter.java

@@ -598,7 +598,7 @@ private void persistMethod(AnalysisMethod method, Supplier<PersistedAnalysisMeth
         }
 
         String methodDescriptor = imageLayerSnapshotUtil.getMethodDescriptor(method);
-        if (methodDescriptors.put(methodDescriptor, method) != null) {
+        if (methodDescriptors.putIfAbsent(methodDescriptor, method) != null) {
             throw GraalError.shouldNotReachHere("The method descriptor should be unique, but %s got added twice.\nThe first method is %s and the second is %s."
                             .formatted(methodDescriptor, methodDescriptors.get(methodDescriptor), method));
         }