fetch: fix duplicate remote parallel fetch bug

Fetching in parallel from a remote group with a duplicated remote results
in the following:

error: cannot lock ref '<ref>': is at <oid> but expected <oid>

This doesn't happen in serial since fetching from the same remote that
has already been fetched from is a noop. Therefore, remove any duplicated
remotes after remote groups are parsed.

Signed-off-by: Calvin Wan <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: calvin-wan-google

builtin/fetch.c

@@ -2228,6 +2228,7 @@ int cmd_fetch(int argc, const char **argv, const char *prefix)
 			argv++;
 		}
 	}
+	string_list_remove_duplicates(&list, 0);
 
 	if (negotiate_only) {
 		struct oidset acked_commits = OIDSET_INIT;

t/t5506-remote-groups.sh

@@ -99,4 +99,13 @@ test_expect_success 'updating remote name updates that remote' '
 	! repo_fetched two
 '
 
+test_expect_success 'updating group in parallel with a duplicate remote does not fail (fetch)' '
+	mark fetch-group-duplicate &&
+	update_repo one &&
+	git config --add remotes.duplicate one &&
+	git config --add remotes.duplicate one &&
+	git -c fetch.parallel=2 remote update duplicate &&
+	repo_fetched one
+'
+
 test_done