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Description
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);