People are highly encouraged to use the search function before posting a suggestion - along with reading the rules. But sadly most don't seem to do either and there's no (practical) way to force them to... There used to be a post dedicated to listing confirmed/planned suggestions, but it seems to have vanished after it became inactive for a while...
I'm not seeing all that many duplicate suggestions though - the main concern are people asking for Pokémon/current Gen features to be added...