diff --git a/dvc/remote/base.py b/dvc/remote/base.py
index 5522711d3..fa85fa170 100644
--- a/dvc/remote/base.py
+++ b/dvc/remote/base.py
@@ -469,10 +469,10 @@ class RemoteBase(object):
     def gc(self, cinfos):
         from dvc.remote.local import RemoteLOCAL
 
-        used = [info[RemoteLOCAL.PARAM_CHECKSUM] for info in cinfos["local"]]
+        used = {info[RemoteLOCAL.PARAM_CHECKSUM] for info in cinfos["local"]}
 
         if self.scheme != "":
-            used += [info[self.PARAM_CHECKSUM] for info in cinfos[self.scheme]]
+            used |= {info[self.PARAM_CHECKSUM] for info in cinfos[self.scheme]}
 
         removed = False
         for checksum in self.all():
diff --git a/dvc/repo/__init__.py b/dvc/repo/__init__.py
index 4d599a71d..4f3f02947 100644
--- a/dvc/repo/__init__.py
+++ b/dvc/repo/__init__.py
@@ -285,12 +285,14 @@ class Repo(object):
 
                 for out in stage.outs:
                     scheme = out.path_info["scheme"]
-                    cache[scheme] += self._collect_used_cache(
-                        out,
-                        branch=branch,
-                        remote=remote,
-                        force=force,
-                        jobs=jobs,
+                    cache[scheme].extend(
+                        self._collect_used_cache(
+                            out,
+                            branch=branch,
+                            remote=remote,
+                            force=force,
+                            jobs=jobs,
+                        )
                     )
 
         return cache