[fix: user-command break]
[fix: user-command break]

	issue: when 'break add' is called multiple times for the same
		   location gdb creates multiple breakpoints while vimgdb does
		   only store the last one. Hence the others cannot be accessed
	fix: check if breakpoint already exists, avoiding multiple creations

jnow-87/vimgdb,jnow-87/vimgdb,jnow-87/vimgdb