diff --git a/scrapy/dupefilters.py b/scrapy/dupefilters.py
index ac5478e7c..292c68099 100644
--- a/scrapy/dupefilters.py
+++ b/scrapy/dupefilters.py
@@ -1,35 +1,47 @@
-import os
 import logging
+import os
+from typing import Optional, Set, Type, TypeVar
 
+from twisted.internet.defer import Deferred
+
+from scrapy.http.request import Request
+from scrapy.settings import BaseSettings
+from scrapy.spiders import Spider
 from scrapy.utils.job import job_dir
 from scrapy.utils.request import referer_str, request_fingerprint
 
 
-class BaseDupeFilter:
+BaseDupeFilterTV = TypeVar("BaseDupeFilterTV", bound="BaseDupeFilter")
 
+
+class BaseDupeFilter:
     @classmethod
-    def from_settings(cls, settings):
+    def from_settings(cls: Type[BaseDupeFilterTV], settings: BaseSettings) -> BaseDupeFilterTV:
         return cls()
 
-    def request_seen(self, request):
+    def request_seen(self, request: Request) -> bool:
         return False
 
-    def open(self):  # can return deferred
+    def open(self) -> Optional[Deferred]:
         pass
 
-    def close(self, reason):  # can return a deferred
+    def close(self, reason: str) -> Optional[Deferred]:
         pass
 
-    def log(self, request, spider):  # log that a request has been filtered
+    def log(self, request: Request, spider: Spider) -> None:
+        """Log that a request has been filtered"""
         pass
 
 
+RFPDupeFilterTV = TypeVar("RFPDupeFilterTV", bound="RFPDupeFilter")
+
+
 class RFPDupeFilter(BaseDupeFilter):
     """Request Fingerprint duplicates filter"""
 
-    def __init__(self, path=None, debug=False):
+    def __init__(self, path: Optional[str] = None, debug: bool = False) -> None:
         self.file = None
-        self.fingerprints = set()
+        self.fingerprints: Set[str] = set()
         self.logdupes = True
         self.debug = debug
         self.logger = logging.getLogger(__name__)
@@ -39,26 +51,27 @@ class RFPDupeFilter(BaseDupeFilter):
             self.fingerprints.update(x.rstrip() for x in self.file)
 
     @classmethod
-    def from_settings(cls, settings):
+    def from_settings(cls: Type[RFPDupeFilterTV], settings: BaseSettings) -> RFPDupeFilterTV:
         debug = settings.getbool('DUPEFILTER_DEBUG')
         return cls(job_dir(settings), debug)
 
-    def request_seen(self, request):
+    def request_seen(self, request: Request) -> bool:
         fp = self.request_fingerprint(request)
         if fp in self.fingerprints:
             return True
         self.fingerprints.add(fp)
         if self.file:
             self.file.write(fp + '\n')
+        return False
 
-    def request_fingerprint(self, request):
+    def request_fingerprint(self, request: Request) -> str:
         return request_fingerprint(request)
 
-    def close(self, reason):
+    def close(self, reason: str) -> None:
         if self.file:
             self.file.close()
 
-    def log(self, request, spider):
+    def log(self, request: Request, spider: Spider) -> None:
         if self.debug:
             msg = "Filtered duplicate request: %(request)s (referer: %(referer)s)"
             args = {'request': request, 'referer': referer_str(request)}
diff --git a/scrapy/utils/request.py b/scrapy/utils/request.py
index 66736b42f..541368423 100644
--- a/scrapy/utils/request.py
+++ b/scrapy/utils/request.py
@@ -24,7 +24,7 @@ def request_fingerprint(
     request: Request,
     include_headers: Optional[Iterable[Union[bytes, str]]] = None,
     keep_fragments: bool = False,
-):
+) -> str:
     """
     Return the request fingerprint.