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1. Chop shrimp into bite-sized pieces and set aside.
2. In a medium bowl, combine onion, garlic, and shrimp. Pour lime and pineapple juice over shrimp and stir. Make sure shrimp are covered by liquid.
3. Cover bowl. Refrigerate 45 minutes, or until shrimp turn opaque.
4. Strain mixture, reserving ½ cup marinating liquid.
5. In the bowl, combine shrimp, ½ cup reserved marinating liquid, mango, tomato, cilantro, salt, and pepper, to taste.
6. Scoop guacamole onto serving plate, top with shrimp mixture.