diff --git a/swh/lister/core/lister_base.py b/swh/lister/core/lister_base.py
--- a/swh/lister/core/lister_base.py
+++ b/swh/lister/core/lister_base.py
@@ -150,6 +150,25 @@
         """
         return models_list
 
+    def do_additional_checks(self, models_list):
+        """Execute some additional checks on the model list. For example, to
+           check for existing repositories in the db.
+
+        MAY BE OVERRIDDEN if an intermediate Lister class needs to
+        check some more the results before injection.
+
+        Checks are fine by default, returns the models_list as is by default.
+
+        Args:
+            models_list: list of dicts returned by
+                         transport_response_simplified.
+
+        Returns:
+            models_list with entries if checks ok, False otherwise
+
+        """
+        return models_list
+
     def is_within_bounds(self, inner, lower=None, upper=None):
         """See if a sortable value is inside the range [lower,upper].
 
@@ -453,18 +472,23 @@
                     [self.task_dict(m['origin_type'], m['origin_url'])]
                 )[0]['id']
 
-    def ingest_data(self, identifier):
+    def ingest_data(self, identifier, checks=False):
         """The core data fetch sequence. Request server endpoint. Simplify and
             filter response list of repositories. Inject repo information into
             local db. Queue loader tasks for linked repositories.
 
         Args:
             identifier: Resource identifier.
+            checks (bool): Additional checks required
         """
         # Request (partial?) list of repositories info
         response = self.safely_issue_request(identifier)
         models_list = self.transport_response_simplified(response)
         models_list = self.filter_before_inject(models_list)
+        if checks:
+            models_list = self.do_additional_checks(models_list)
+            if not models_list:
+                return response, []
         # inject into local db
         injected = self.inject_repo_data_into_db(models_list)
         # queue workers
diff --git a/swh/lister/core/paging_lister.py b/swh/lister/core/paging_lister.py
--- a/swh/lister/core/paging_lister.py
+++ b/swh/lister/core/paging_lister.py
@@ -79,14 +79,18 @@
 
     # You probably don't need to override anything below this line.
 
-    def check_existence(self, injected_repos):
-        """Given a list of injected repos, check if we already have them.
+    def do_additional_checks(self, models_list):
+        """Potentially check for existence of repositories in models_list.
 
-        Attribute 'instance' variable is assumed to be populated.
+        This will be called only if check_existence is flipped on in
+        the run method below.
 
         """
-        # FIXME: Implement the check
-        return False
+        for m in models_list:
+            sql_repo = self.db_query_equal('uid', m['uid'])
+            if sql_repo:
+                return False
+        return models_list
 
     def run(self, min_bound=None, max_bound=None, check_existence=False):
         """Main entry function. Sequentially fetches repository data from the
@@ -111,16 +115,17 @@
 
         self.min_page = min_bound
         self.max_page = max_bound
-        already_seen = False
 
         while self.is_within_bounds(page, self.min_page, self.max_page):
             logging.info('listing repos starting at %s' % page)
 
-            response, injected_repos = self.ingest_data(page)
-            next_page = self.get_next_target_from_response(response)
+            response, injected_repos = self.ingest_data(page,
+                                                        checks=check_existence)
+            if not injected_repos:
+                logging.info('Repositories already seen, stopping')
+                break
 
-            if check_existence:
-                already_seen = self.check_existence(injected_repos)
+            next_page = self.get_next_target_from_response(response)
 
             # termination condition
 
@@ -128,9 +133,6 @@
                 logging.info('stopping after page %s, no next link found' %
                              page)
                 break
-            elif already_seen:
-                logging.info('Repositories already seen, stopping')
-                break
             else:
                 page = next_page