SR197: Non unique primary key on DB.DBA.SYS_XML_PERSISTENT_NS_DECL

Question

I am trying to upload ttl files to Virtuoso. There are around 180 files being uploaded to a single graph in a parallel fashion (parallel stream in java), and sometimes I get this exception and the uploading fails after few files have been uploaded. This happens sometimes, and other times the uploading finished successfully with no error or exception. Below is the exception with the stack trace that I encounter.

    SR197: Non unique primary key on DB.DBA.SYS_XML_PERSISTENT_NS_DECL. 
org.apache.jena.shared.JenaException: org.apache.jena.shared.JenaException: virtuoso.jdbc4.VirtuosoException: SR197: Non unique primary key on DB.DBA.SYS_XML_PERSISTENT_NS_DECL.
    at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:77)
    at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.base/java.lang.reflect.Constructor.newInstanceWithCaller(Constructor.java:499)
    at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:480)
    at java.base/java.util.concurrent.ForkJoinTask.getThrowableException(ForkJoinTask.java:562)
    at java.base/java.util.concurrent.ForkJoinTask.reportException(ForkJoinTask.java:591)
    at java.base/java.util.concurrent.ForkJoinTask.invoke(ForkJoinTask.java:689)
    at java.base/java.util.stream.ForEachOps$ForEachOp.evaluateParallel(ForEachOps.java:159)
    at java.base/java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateParallel(ForEachOps.java:173)
    at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:233)
    at java.base/java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:596)
    at java.base/java.util.stream.ReferencePipeline$Head.forEach(ReferencePipeline.java:765)
    at com.elsevier.reaxys.loader.TripleStoreLoaderApp.getFilesAndUpload(TripleStoreLoaderApp.java:65)
    at com.elsevier.reaxys.loader.TripleStoreLoaderApp.run(TripleStoreLoaderApp.java:49)
    at org.springframework.boot.SpringApplication.callRunner(SpringApplication.java:762)
    at org.springframework.boot.SpringApplication.callRunners(SpringApplication.java:752)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:315)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1306)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1295)
    at com.elsevier.reaxys.loader.TripleStoreLoaderApp.main(TripleStoreLoaderApp.java:35)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:568)
    at org.springframework.boot.loader.MainMethodRunner.run(MainMethodRunner.java:49)
    at org.springframework.boot.loader.Launcher.launch(Launcher.java:108)
    at org.springframework.boot.loader.Launcher.launch(Launcher.java:58)
    at org.springframework.boot.loader.JarLauncher.main(JarLauncher.java:65)

Can someone guide me what this error means and how can I avoid it? Thank You in advance!!