A free trade agreement or customs union can reduce or eliminate duty on qualifying goods moving between member or partner countries, but the preference is never automatic. You must prove origin with the correct certificate, and the product must meet the specific origin rule set out in that agreement, which can be stricter than a general 'made in' claim.
Because these agreements change and expand over time, check current coverage before assuming a lower rate applies, rather than relying on what was true when you last imported.