d = {'a': [10, 15], 'b': [10, 20], 'c': [10, 15]}
values = []
for k, v in d.items():
    must_insert = True
for val in values:
    if val[0] == v:
        val[1].append(k)
must_insert = False
break
if must_insert:
    values.append([v, [k]])
print([v for v in values if len(v[1]) > 1])
How to check if two keys in dict hold the same value