How to avoid double taxation in Spain?

Could you kindly explain how I can effectively avoid double taxation in Spain? I've heard that it's a common issue, especially for those who hold assets or earn income in multiple countries. Is there a specific strategy or set of rules I should follow to ensure I'm not taxed twice on the same income? Also, I'm curious about the tax treaties Spain has with other countries. Could these treaties help me avoid double taxation? Lastly, are there any specific tax shelters or loopholes I should be aware of? Thank you for your assistance in clarifying this matter.