diff --git a/scrapy/trunk/scrapy/core/filters.py b/scrapy/trunk/scrapy/core/filters.py
new file mode 100644
index 000000000..9dc212ab8
--- /dev/null
+++ b/scrapy/trunk/scrapy/core/filters.py
@@ -0,0 +1,86 @@
+from pydispatch import dispatcher
+
+from scrapy.core import signals
+from scrapy.http import Request
+from scrapy.core.exceptions import NotConfigured, IgnoreRequest
+from scrapy.utils.request import request_fingerprint
+from scrapy.utils.misc import load_object
+from scrapy.conf import settings
+from scrapy import log
+
+
+class BaseFilter(dict):
+    """Base class defining the duplicates requests filtering api"""
+
+    def __init__(self, *args, **kwargs):
+        super(BaseFilter, self).__init__(*args, **kwargs)
+        dispatcher.connect(self.open, signals.domain_open)
+        dispatcher.connect(self.close, signals.domain_closed)
+
+    def open(self, domain):
+        """Called when a domain starts"""
+        raise NotImplementedError()
+
+    def close(self, domain):
+        """Called when a domain is closed"""
+        raise NotImplementedError()
+
+    def add(self, domain, request):
+        """Called to check if a request was already seen, and adds it to seen set.
+
+        returns True if not seen before, or False otherwise.
+        """
+        raise NotImplementedError()
+
+    def has(self, domain, request):
+        """Called to check if a request was seen but doesnt add request to seen set."""
+        raise NotImplementedError()
+
+
+class SimplePerDomainFilter(BaseFilter):
+    """Filter out a request if already seen for same domain"""
+
+    def open(self, domain):
+        """Initialize the resources needed for filtering for this domain"""
+        self[domain] = set()
+
+    def close(self, domain):
+        """Remove the resources reserved for filtering for this domain"""
+        del self[domain]
+
+    def add(self, domain, request):
+        """Add the fingerprint of a request to the domain set if a equivalent fingerprint has not been added.
+        This method will return true if the fingerprint was added and false otherwise.
+        """
+        fp = request_fingerprint(request)
+        if fp not in self[domain]:
+            self[domain].add(fp)
+            return True
+        return False
+
+    def has(self, domain, request):
+        fp = request_fingerprint(request)
+        return fp in self[domain]
+
+
+class NullFilter(dict):
+    def open(self, domain):
+        pass
+
+    def close(self, domain):
+        pass
+
+    def add(self, domain, request):
+        return True
+
+    def has(self, domain, request):
+        return None
+
+
+try:
+    duplicatesfilter
+except NameError:
+    clspath = settings.get('DUPLICATESFILTER_FILTERCLASS')
+    cls = load_object(clspath) if clspath else NullFilter
+    duplicatesfilter = cls()
+
diff --git a/scrapy/trunk/scrapy/tests/test_filters.py b/scrapy/trunk/scrapy/tests/test_filters.py
new file mode 100644
index 000000000..ba71b9614
--- /dev/null
+++ b/scrapy/trunk/scrapy/tests/test_filters.py
@@ -0,0 +1,41 @@
+import unittest
+
+from scrapy.http import Request
+from scrapy.core.filters import SimplePerDomainFilter, NullFilter
+
+
+class SimplePerDomainFilterTest(unittest.TestCase):
+
+    def test_filter(self):
+        domain = 'scrapytest.org'
+        filter = SimplePerDomainFilter()
+        filter.open(domain)
+        assert domain in filter
+
+        r1 = Request('http://scrapytest.org/1')
+        r2 = Request('http://scrapytest.org/2')
+        r3 = Request('http://scrapytest.org/2')
+
+        assert not filter.has(domain, r1)
+        assert filter.add(domain, r1)
+        assert filter.has(domain, r1)
+
+        assert filter.add(domain, r2)
+        assert not filter.add(domain, r3)
+
+        filter.close(domain)
+        assert domain not in filter
+
+
+class NullFilterTest(unittest.TestCase):
+
+    def test_filter(self):
+        domain = 'scrapytest.org'
+        filter = NullFilter()
+        filter.open(domain)
+
+        r1 = Request('http://scrapytest.org/1')
+        assert not filter.has(domain, r1)
+        assert filter.add(domain, r1)
+        assert not filter.has(domain, r1)
+        filter.close(domain)