diff --git a/dhp-pace-core/src/main/java/eu/dnetlib/pace/util/BlockProcessor.java b/dhp-pace-core/src/main/java/eu/dnetlib/pace/util/BlockProcessor.java
index fae749383..95918a7c7 100644
--- a/dhp-pace-core/src/main/java/eu/dnetlib/pace/util/BlockProcessor.java
+++ b/dhp-pace-core/src/main/java/eu/dnetlib/pace/util/BlockProcessor.java
@@ -58,7 +58,7 @@ public class BlockProcessor {
 		this.orderFieldPos = orderFieldPos;
 	}
 
-	public void processSortedRows(final Collection<Row> documents, final Reporter context) {
+	public void processSortedRows(final List<Row> documents, final Reporter context) {
 		if (documents.size() > 1) {
 //            log.info("reducing key: '" + key + "' records: " + q.size());
 			processRows(documents, context);
@@ -68,13 +68,10 @@ public class BlockProcessor {
 		}
 	}
 
-	private void processRows(final Collection<Row> queue, final Reporter context) {
+	private void processRows(final List<Row> queue, final Reporter context) {
 
-		Iterator<Row> it = queue.iterator();
-		while (it.hasNext()) {
-
-			final Row pivot = it.next();
-			it.remove();
+		for (int pivotPos = 0; pivotPos < queue.size(); pivotPos++) {
+			final Row pivot = queue.get(pivotPos);
 
 			final String idPivot = pivot.getString(identifierFieldPos); // identifier
 			final Object fieldsPivot = getJavaValue(pivot, orderFieldPos);
@@ -83,13 +80,12 @@ public class BlockProcessor {
 
 			if (fieldPivot != null) {
 				int i = 0;
-				for (final Row curr : queue) {
+				for (int windowPos = pivotPos + 1; windowPos < queue.size(); windowPos++) {
+					final Row curr = queue.get(windowPos);
 					final String idCurr = curr.getString(identifierFieldPos); // identifier
 
 					if (mustSkip(idCurr)) {
-
 						context.incrementCounter(wf.getEntityType(), "skip list", 1);
-
 						break;
 					}
 
@@ -105,7 +101,6 @@ public class BlockProcessor {
 						final TreeProcessor treeProcessor = new TreeProcessor(dedupConf);
 
 						emitOutput(treeProcessor.compare(pivot, curr), idPivot, idCurr, context);
-
 					}
 				}
 			}