Not all starships that carry the same name have the same Starfleet registry with a letter appended at the end (like the Enterprises do).
There have been three Federation starships named USS Defiant depicted on screen and they each have a unique registry number:
NCC-1764 Constitution Class [TOS, ENT]
NX-74205 Defiant Class (class prototype) [DS9]
NCC-75633 Defiant Class (originally USS Sao Paulo) [DS9]
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