diff --git a/dhp-workflows/dhp-graph-mapper/src/main/java/eu/dnetlib/dhp/oa/graph/raw/OdfToOafMapper.java b/dhp-workflows/dhp-graph-mapper/src/main/java/eu/dnetlib/dhp/oa/graph/raw/OdfToOafMapper.java
index 62f8123bb..fa0e5221d 100644
--- a/dhp-workflows/dhp-graph-mapper/src/main/java/eu/dnetlib/dhp/oa/graph/raw/OdfToOafMapper.java
+++ b/dhp-workflows/dhp-graph-mapper/src/main/java/eu/dnetlib/dhp/oa/graph/raw/OdfToOafMapper.java
@@ -12,6 +12,7 @@ import java.util.HashSet;
 import java.util.List;
 import java.util.Set;
 
+import com.google.common.collect.Lists;
 import org.apache.commons.lang3.StringUtils;
 import org.dom4j.Document;
 import org.dom4j.Node;
@@ -366,7 +367,7 @@ public class OdfToOafMapper extends AbstractMdRecordToOafMapper {
 
 	@Override
 	protected List<StructuredProperty> prepareResultPids(final Document doc, final DataInfo info) {
-		final List<StructuredProperty> res = new ArrayList<>();
+		final Set<StructuredProperty> res = new HashSet();
 		res
 			.addAll(
 				prepareListStructPropsWithValidQualifier(
@@ -382,7 +383,7 @@ public class OdfToOafMapper extends AbstractMdRecordToOafMapper {
 					doc,
 					"//datacite:alternateIdentifier[@alternateIdentifierType != 'URL' and @alternateIdentifierType != 'landingPage']",
 					"@alternateIdentifierType", DNET_PID_TYPES, info));
-		return res;
+		return Lists.newArrayList(res);
 	}
 
 }