In a frying pan, saute the chopped onion and shallots in butter. Add the shrimp and cook shrimp until pink in color. Add chopped parsley and capers to the pan. Remove the pan from the heat and pour in half of the tequila. Return the pan to the heat. Season with salt and pepper, to taste. When ready to serve, remove the pan from the heat and pour in the rest of the tequila. Using a match, flambe the alcohol carefully. Flames should die down in a few seconds. Serve hot.