Writing Libraries that use libraries that are being used in the sketch
The proper way to handle this is to have an #include guard in the header file to prevent the content from being included more than once.

In your header file filename.h you would add something like this:

#ifndef _FILENAME_H_
#define _FILENAME_H_

// Your header code here

#endif /* _FILENAME_H_ */


The #ifndef block will prevent the code from being included more than once. Best practice is to always put include guards in header files.