EXTADT in libextobjc

Question

So I'm trying use this library in my iOS app using objective-c:

https://github.com/jspahrsummers/libextobjc/blob/master/extobjc/EXTADT.h

which is included in the libextobjc package, installable via cocoapods. Say I create some ADT and name this "SomeADT.h". I just basically follow the example in the documentation.

Say I include SomeADT.h in SomeFile1.m and use it. It compiles fine.

For some reason, however, when I include SomeADT.h also in SomeFile2.m , it gives me an error of this sort:

duplicate symbol _SomeADT in: /Users/.../SomeFile1.o /Users/.../SomeFile2.o ld: 1 duplicate symbol for architecture i386 clang: error: linker command failed with exit code 1 (use -v to see invocation)

The SomeADT.h file has ifndef macro etc., why would it still complain about this? I feel like I'm missing something quite obvious here. For reference, I don't have a SomeADT.c file, but it doesn't seem necessary.