Can the maybe_unused attribute specifier appear only once and still work?

Question

I like the [[maybe_unused]] attribute and use it a lot when I need to compile out debug-only function implementations. I am a little confused where it should be used when specifying member function arguments as [[maybe_unused]].

Given:

//Foo.h
class Foo {
    void DebugOnly(Bar& fly);
}

//Foo.cpp
void Foo::DebugOnly(Bar& fly) {
#ifdef _DEBUG
    fly.squish();
#endif
}

Does specifying it in both places make a difference?

//Foo.h
class Foo {
    void DebugOnly([[maybe_unused]] Bar& fly);
}

//Foo.cpp
void Foo::DebugOnly([[maybe_unused]] Bar& fly) {
#ifdef _DEBUG
    fly.squish();
#endif
}

Or should I only declare it in the header or implementation file?

Header-only

//Foo.h
class Foo {
    void DebugOnly([[maybe_unused]] Bar& fly);
}

//Foo.cpp
void Foo::DebugOnly(Bar& fly) {
#ifdef _DEBUG
    fly.squish();
#endif
}

Implementation-file-only

//Foo.h
class Foo {
    void DebugOnly(Bar& fly);
}

//Foo.cpp
void Foo::DebugOnly([[maybe_unused]] Bar& fly) {
#ifdef _DEBUG
    fly.squish();
#endif
}

The page on cppreference does not make a distinction for function arguments and only classifies them as "variables".