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I've heard that most snipes are water based glues, so I would think that spraying/soaking only the portion of the poster that has the snipe will loosen it up if the glue is water based. place a soaked paper towel underneath, and on top, and keep it soaked for 10 mins? take a sharp knife and gently, ever so carefully try to lift a corner off. If it's water based it should slide right off. If not, try for a 5 more mins.
If unsuccessful, then I'd try UnDu, or in the states we use Bestine. Good luck!