Note that 'C' is case-sensitive, and your program's entry point must be named 'main' - with a lowercase initial 'm'!!


That will be because your header file contains definitions of the variables; therefore, each file that includes it will contain those definitions.
Because you include it in multiple (ie, more than one) files, that will mean that your project will have multiple definitions - just as the message says!

You header file should contain only declaration, and each variable should be defined in one - and only one - .c file.

This is standard 'C' textbook stuff - nothing specifically to do with Keil nor the 8051!
See: http://c-faq.com/decl/decldef.html