Multiple definition errors when including WidgetTimeInput.h in separate class

I want to build a wake up light using a Raspberry Pi. Therefore I want to make a separate class to convert WidgetTimeInput to a crontab line.

In that class I want to include WidgetTimeInput.h. However doing so throws multiple definition errors at compile time. This one of them:

main.o: In function blynk_count_millis()': main.cpp:(.text+0x24): multiple definition of blynk_count_millis()’
./src/WakeUpLight.o:WakeUpLight.cpp:(.text+0x0): first defined here

All off these functions are defined in blynk-library/src/Blynk/BlynkEveryN.h
These functions are already in an ifndef construct.

Maybe this problem is related to this one [SOLVED] Multiple #include across IDE tabs causing multiple definition errors - #20 by JRobert but I couldn’t find a solution there.

My question:
How can I overcome these errors?