Strange Multiple Definition Error using makefile

[LeqxLeqx via D.gnu]

Hello,

When compiling a simple project with makefile, I'm getting the 
error:
myclass.d:(.text+0x88): multiple definition of 
`_DT16_D15myabstractclass15MyAbstractClass11firstMethodMFZv'

I'm not certain what's actually wrong with my code. The error 
occurs when the all of the original object (*.o) files are linked 
together to make the executable. According to `nm', both the 
files `myabstractclass.o' and `myclass.o' contain a definition of:

0000000000XXXXXX T 
_DT16_D15myabstractclass15MyAbstractClass11firstMethodMFZv

(where the X's are replaced by their offset in the file)

I've distilled the error to the smallest number of files and 
lines that seem to induce it. The 4 source files are as follows:

============ main.d ==============

    module main;
    import myclass;

    int main(string[] args)
    {
      MyClass myClass;
      myClass = new MyClass;
      myClass.firstMethod;
      myClass.secondMethod;
      return 0;
    }


======== myinterface.d ============

    module myinterface;

    interface MyInterface
    {
      void firstMethod();
      void secondMethod();
    }

======= myabstractclass.d =========

    module myabstractclass;
    import myinterface;

    abstract class MyAbstractClass : MyInterface
    {
      void firstMethod()
      {
        import std.stdio : writefln;
        writefln("hello from firstMethod");
      }
    }

============ myclass.d ==========

    module myclass;

    import myabstractclass;

    class MyClass : MyAbstractClass
    {
      void secondMethod()
      {

      }
    }

===== and my simple makefile ======

    O_FILES=myinterface.o myabstractclass.o myclass.o main.o
    CC=gdc

    .PHONY : all
    all : $(O_FILES)
            $(CC) $(O_FILES) -o test.bin

    %.o : %.d
            $(CC) -c -o $@ $<

    .PHONY : clean
    clean :
            rm -f $(O_FILES) test.bin

=================================

I'm using GDC version: `gdc (Ubuntu 8.2.0-1ubuntu2~18.04) 8.2.0'

So ultimately, my question is: What am i doing wrong here?
Any and all help is appreciated.