What do you mean by ‘double holo’? A.f.a.i.k., the double holo is a term used for the Neo Revelation set holos with additional holo parts (e.g. Misdreavus’ eyes):
I have seen a ‘double holo’ in the Neo Discovery set once, which is Scizor:
But I don’t know of any double holos for the Neo Genesis set.
Also, looking at the guide I’ve linked, all minor double holos in that guide are missing the white outlines around the Pokémon, which the Slowking in your picture has:
The Slowking definitely looks pretty clean in that single picture. I don’t know the current value to say whether 175 or 275 or either is a decent or normal price for it. But that Slowking isn’t a double holo..
Greetz,
Quuador