Engines hold filter duplicates

[chrmod]

While testing FiltersEngine.merge we've noticed that engines are able to keep duplicated filters.

Steps to reproduce:

const engineBlob = fs.readFileSync(pathToEngine);
const engine = FiltersEngine.deserialise(engineBlob); 
const emptyEngine = FiltersEngine.empty(engine.config);
const newEngine = FiltersEngine.merge([engine, emptyEngine]);
assert(engine.getFilters().networkFilters.length === newEngine.getFilters().networkFilters.length);
assert(engine.getFilters().cosmeticFilters.length === newEngine.getFilters().cosmeticFilters.length);