To illustrate the problem, we use a contrived program, consisting three components: main.cpp as the driver, and a pair of src1 and src2.

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// main.cpp
#include "src1.h",
#include "src2.h"

int main() {
    f1();
    f2();
    return 0;
}
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// src1.h

#pragma once

void f1();
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// src1.cpp
#include <iostream>

struct Test {
    Test() {
        std::cout << "src1 Test\n";
    }
};

void f1() {
    Test t;
}
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// src2.h

#pragma once

void f2();
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// src2.cpp
#include <iostream>

struct Test {
    Test() {
        std::cout << "src2 Test\n";
    }
};

void f2() {
    Test t;
}
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$ clang++ hello.cc src1.cpp src2.cpp ; ./a.out
src1 Test
src1 Test

Note the msg for f2 is incorrect. A simple fix is to place the struct inside the function, so that both struct have internal linkage. For example for src1.cpp:

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// src1.cpp
#include <iostream>

void f1() {
    struct Test {
        Test() {
            std::cout << "src1 Test\n";
        }
    };

    Test t;
}
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$ clang++ hello.cc src1.cpp src2.cpp ; ./a.out
src1 Test
src2 Test

This kind of bug is subtle and it has its counterpart in C as well – functions defined at top-level have external linkage. The quick fix in C is to mark them as static.