diff --git a/onionrblocks/universalrules.py b/onionrblocks/universalrules.py
index f7b2e15..9c02c35 100644
--- a/onionrblocks/universalrules.py
+++ b/onionrblocks/universalrules.py
@@ -2,6 +2,9 @@ from kasten import Kasten
 
 from time import time
 from math import floor
+from typing import Iterable
+
+import binascii
 
 MAX_FUTURE_SKEW_SECONDS = 60 * 2
 TOTAL_MAX_SIZE = 6 * (10**6)
@@ -11,17 +14,25 @@ class BlockRulesException(Exception):
     pass
 
 
-class BlockExpired(Exception):
+class BlockExpired(BlockRulesException):
     pass
 
 class BlockFutureSkewedBeyondMax(BlockRulesException):
     pass
 
+class BlockExistsError(BlockRulesException):
+    pass
 
 class BlockTooLarge(BlockRulesException):
     pass
 
 
+def checksum_exists_in_list(checksum: bytes, arr: 'Iterable'):
+    if checksum in arr:
+        raise BlockExistsError(
+            binascii.hexlify(checksum).decode("utf-8") + " Already exists in given set")
+
+
 def check_block_sanity(raw_bytes):
     kasten: Kasten = Kasten(None, raw_bytes, None, auto_check_generator=False)
 
diff --git a/tests/test_universal_rules.py b/tests/test_universal_rules.py
index 60fb1bf..90276df 100644
--- a/tests/test_universal_rules.py
+++ b/tests/test_universal_rules.py
@@ -1,5 +1,6 @@
 import unittest
 from onionrblocks import universalrules
+from onionrblocks.generators import anonvdf
 import kasten
 
 from time import time
@@ -38,4 +39,16 @@ class TestUniversalRules(unittest.TestCase):
             universalrules.BlockExpired,
             universalrules.check_block_sanity, packed)
 
+    def test_not_in_set(self):
+        t = floor(time()) - 6
+        bls = []
+        for i in range(3):
+            packed = kasten.generator.pack.pack(b"1"*2*i, b"tst", 0, timestamp=t)
+            k = anonvdf.AnonVDFGenerator.generate(packed, 1000)
+            bls.append(k.id)
+        packed = kasten.generator.pack.pack(b"1"*2*1, b"tst", 0, timestamp=t)
+        k = anonvdf.AnonVDFGenerator.generate(packed, 1000)
+        self.assertRaises(universalrules.BlockExistsError, universalrules.checksum_exists_in_list, k.id, bls)
+
+
 unittest.main()
\ No newline at end of file