My solution: OK

Test case #3
found: a "node a"
found: b "node b"
found: c "node c"
My solution: OK

Test case #4
found: 0 "node #0"
found: 3 "node #3"
found: 1 "node #1 with very slow fetching"
found: 2 "node #2"
found: 4 "node #4"
found: 6 "node #6"
found: 5 "node #5"
My solution: OK

Test case #5
found: 0 "node #0"
found: 3 "node #3"
found: 1 "node #1"
found: 4 "node #4"
found: 5 "node #5"
found: 2 "node #2"
fatal error: concurrent map read and map writefound: 13 "node #13"

type warningFetcher struct {
        fetcher        crawler.Fetcher
        alreadyFetched map[string]bool
        lock           sync.Mutex
}

func (f warningFetcher) Fetch(url string) (string, []string, error) {
        f.lock.Lock()
        defer f.lock.Unlock()
        if f.alreadyFetched[url] {
                fmt.Printf("WARNING: Url %s has been fetched multiple times\n", url)
        }
...

-func (f warningFetcher) Fetch(url string) (string, []string, error) {
+func (f *warningFetcher) Fetch(url string) (string, []string, error) {
        f.lock.Lock()
        defer f.lock.Unlock()
        if f.alreadyFetched[url] {
@@ -40,5 +41,5 @@ func (f warningFetcher) Fetch(url string) (string, []string, error) {
 }

 func newWarningFetcher(fetcher crawler.Fetcher) crawler.Fetcher {
-       return warningFetcher{fetcher: fetcher, alreadyFetched: make(map[string]bool)}
+       return &warningFetcher{fetcher: fetcher, alreadyFetched: make(map[string]bool)}