I work in Hong Kong. My work visa states "good for a single journey to Hong kong" which was stamped as -journey completed- when I entered Hong Kong in last august.

The "single journey" refers to your initial arrival to HK for residence / work (i.e. visa activation). All HK employment visas say that. On arrival, your passport will be marked "Allowed to stay till Date xxxxx". You can travel freely in Hong Kong during that time.

Once you're in HK, you will be required to apply for a Hong Kong ID card within 30 days of your arrival. Once you receive the card, you can make multiple entries and use that as your immigration document when you return to Hong Kong (while your visa is valid). Thus, once your visa is activated and you have a Hong Kong ID, you will not receive any more Hong Kong immigration stamps in your passport until your employment visa expires.