I need to create a method that takes a Map and returns a Map where the keys and values are replaced by places

Question

E.g. key: value:

10 value1

20 value2

30 value1

the result would be

value1 {10, 30}

value2. {20}

I am trying to write method which would compare all values and rewrite the value of result map(rev):

public static <K, V> Map<V, K> reverse(Map<K, V> currency) {
    Map<V, K> rev = new HashMap<>();
    for (Map.Entry<K, V> entry : currency.entrySet()) {
        for (Map.Entry<K, V> entry1 : currency.entrySet()) {
            if (entry.getValue() == entry1.getValue()) rev.replace(entry.getValue(), __ + entry.getKey());
            else rev.put(entry.getValue(), entry.getKey());
        }
    }
    return rev;
}

Not sure how can I find previous value in rev map. Please help!