this code appears to respond twice.

 

I assume once is from the Client and once from the server?

 

	@Override
    public boolean onBlockActivated(World worldIn, BlockPos pos, IBlockState state, EntityPlayer playerIn, EnumHand hand, @Nullable ItemStack heldItem, EnumFacing side, float hitX, float hitY, float hitZ)
    {
	System.out.println("clicked");
	return true;
    }

Block#onBlockActivated

can be called up to four times per click: once per hand on both the client and server.

 

Return

true

when an action has been taken to prevent the next hand from being processed; return

false

if nothing was done and the next hand should be processed.