Answer to Philosophy1, copied here because others may have a similar concern:
Your fear of paying double is unfounded. If you pay for 2 month, and still have 15 days left, the price for the 15 days will be substracted from the price you have to pay.
If you start the buying process, the last thing on the Dominion Online website will be a list wich says (paraphrased)
Gold subscription: 7.00€
Already subscribed: -1.70€
The -1.70€ is the value if your remaining subscription for 15 days.