diff --git a/src/server.py b/src/server.py
--- a/src/server.py
+++ b/src/server.py
@@ -33,9 +33,12 @@ nodeStack=collections.deque([0])
 incoming=networkReader.output # synchronized message queue
 outcoming=networkWriter.input
 
+i1=-1
+
 
 while True:
 	jsonData,binData=incoming.get(timeout=2)
+	dataFile=open(filename,mode="rb+")
 	
 	if jsonData["command"]=="init":
 		assert jsonData["blockSize"]==localTree.BLOCK_SIZE
@@ -52,11 +55,12 @@ while True:
 	
 	elif jsonData["command"]=="send" and jsonData["dataType"]=="data": # needlessly allow hashes and data in mixed order
 		print("received data block #{0}: {1}...{2}".format(jsonData["index"],binData[:5],binData[-5:]))
-		
-		dataFile=open(filename,mode="rb+")
-		dataFile.seek(jsonData["index"]*localTree.BLOCK_SIZE)
+
+		i2=jsonData["index"]
+		if i1+1!=i2:
+			dataFile.seek(i2*localTree.BLOCK_SIZE)
 		dataFile.write(binData)
-		dataFile.close()
+		i1=i2
 		
 		# never update the hash tree
 		
@@ -68,5 +72,6 @@ while True:
 
 # fr.close()
 # fw.close()
+dataFile.close()
 conn.close()
 sys.exit(0)